First things first: Choosing our ruler. For what we’re going to do, we will start at the Stamford Bridge bookmark (15 September 1066) and choose Muchard ua Briain, the Petty King of Mumu, in southern Ireland. We’re choosing him, even though he’s not that great of a character, for three main reasons:

• He has only 1 feudal vassal (Ormound), who is very easy to control (or you can even arrest and banish him if you don't mind tyranny)
  
• He starts with 3 claims, one of which is de jure and right next door in Desmond
  
• His territory, Thormund, produces more income than most others in Ireland

Now that we’ve selected our ruler and loaded the game, let’s assess our situation. Muchard is unmarried, has 2 half-brothers and a son, who is also unmarried. Set your ambition to “Get Married” and find a suitable bride. For right now, I like to marry for stats that Muchard lacks. You can also marry for alliances or claims, but we’ll do that later. Marry Muchard and boom, free 100 Prestige and also the choice of more prestige or gold. Early on, I usually go with the money.
Now we need to find a bride for our son and heir. Here’s where things can get complicated. You have two paths go to down here: marry for an alliance with a strong European power (usually France has available brides) or marry for claims. If you’re looking for an alliance, choose a daughter or sister of a powerful King or Duke (powerful meaning how many troops they have) that isn’t too far away so that they can help you in wars. I really like the latter path, but keep in mind that the fruits of this can take years to come to bear. If you want to marry your son for claims, I recommend the following: Sort your list by “Culture” and look for Welsh brides. Choose a bride that has claims that can be inherited. This is vital. Years later, you want to be able to press claims in Wales on behalf of your dynasts. I’ll get more into this later. Another thing to keep in mind in general when choosing brides/husbands is if they have the Lustful trait, as it means that you’re likely to have lots of kids - this is a good thing and you want this. Children of rulers/rulers’ heirs mean alliances and children from strategically-made marriages mean claims that can be pressed later. There are many other things you could consider when choosing spouses, but that’s more advanced gameplay and this is a basics guide.

Now that we’ve dealt with our family matters, let’s check out our Council. Your two most important councilors by far are you Chancellor and Spymaster. The former handles your diplomacy and helps you expand, and the latter helps you deal with pots against your rule. As a general rule, 10 is the lowest acceptable ability level for a councilor, but ideally you don’t want anyone lower than 15. If you have a councilor lower than 10, replace them immediately. If there’s no one at your court who is suitable to the job, you can look abroad in this manner: Go to the Character Search screen Use the following criteria:

• Male
  
• Not imprisoned
  
• Not ruler
  
• My religious group
  
• Adult

Look for people with claims, indicated by the crown on their portrait, as they are the only people who will accept an invitation to your court. The idea behind this mechanic is that these are individuals who have claims on titles and are, for whatever reason, extremely unlikely to claim or inherit those titles themselves. So what you are doing is saying “Hey, if you come here, I’ll press that claim on your behalf.” Of course, you never actually do this, but it’s how the game works.
Now that you have a competent council, assign them the following jobs:

• Chancellor: if the Earl of Osmound doesn’t like you, Improve Relations with him. If he does, Fabricate Claim on Kildare
  
• Marshall: Train Troops in your capital
  
• Steward: Collect Taxes in your capital
  
• Spymaster: Build Spy Network in your capital or Research Technology in Paris, France or Nassau, Holy Roman Empire
  
• Court Chaplain: Improve Religious Relations with the Pope in Rome.

The temptation with your Councillors is to have them research technology in your capital. Do not do this - it’s extremely unproductive and you’re giving up either money or troops to do so. Generally speaking, you’ll keep your Steward, and Marshall in your capital doing their respective things; your Court Chaplain in Rome making the Pope like you (unless heresies spring up in your realm and your Court Chaplain will have to deal with those) and will be moving around your Chancellor and Spymaster as needed. Your Marshall is a little more complicated but again, this is a basics guide.
Now our family is straightened out and our Councillors are doing their good work. So much work and we haven’t even pressed play yet!

Before we set out conquering Ireland, a quick note. It’s extremely likely that Muchard will die before you finish uniting Ireland, and also that his son won’t do it either, taking 3 generations to finish the task. This is normal. Do not worry if Muchard dies before you create Ireland. Every time I do it, it’s my grandson that finishes the job.

Ok, that’s out of the way. Set your ambition to “Become King of Ireland” and note that you have 3 claims that you can press on day 1: the counties of Desmond, Tyrone and Oriel. Desmond is where we will start, as it is de jure part of the Petty Kingdom of Mumu. Click on the County of Desmond and then the Earl’s portrait. On the right hand side, you'll see his current stats (gold, troops, prestige, piety, etc) and note how many troops he has (represented by the helmet). Do the same for yourself, and if you have more troops than him, go ahead and declare war. If not, it’s because your vassal, the Earl of Osmond, doesn’t like you. Improve Relations with him before you declare war. Or, if you don't mind a bit of tyranny, arrest and banish him. Once you declare war and/or improve relations with the Earl of Osmond, move your Chancellor to Kildare and fabricate a claim on that. Crusader Kings II requires a lot of multitasking; get used to it.

When you declare war, you are saying to the Earl of Desmond “Accept me as your liege!” thus your victory will NOT make you the Earl of Desmond: the Earl of Desmond will just become your vassal. When you declare war, he will immediately raise his army and almost always march his troops to your capital. This is good, because:

• You have more troops than him
  
• There’s a river between Thormund (your capital county) and Desmond, so his army will take a significant penalty for crossing it.

Defeat his army, siege Desmond and defeat the Earl. You have united the entire Petty Kingdom of Mumu and it’s time to move on to the rest of Ireland. Fabricate claims on and conquer the counties of Kildare (the weakest nearby), Ossory, Leinister, and Dublin. Muchard and his son will also have claims on the counties of Tyrone and Oriel and these are very weak when they’re independent. You can nab them too. When you control 51% of the territory of Ireland, you can create the title King of Ireland for yourself. Now you’re the King of Ireland, but only in name. We have more work to do.

Once you become King of Ireland, you can offer vassalization to any Earl that you haven’t conquered that is part of the de jure Kingdom of Ireland, and most will accept. Sometimes, an Earl won’t accept but when those around him do, he’ll change his mind. I find that the Duke of Connact usually needs to be conquered, but you have so many more troops than him that it’s really no big deal. Hooray, now you’re REALLY the King of Ireland! Pause and have a Guinness!

So, remember how I said that Crusader Kings II requires a lot of multitasking? Well, while you’re out conquering Ireland, there a few other things that you need to be doing at the same time. First, you need to upgrade your holdings in your capital county of Thormund. In most cases, you want to prioritize economic buildings over military buildings, since you can hire mercenaries if you have the money but you can’t loot or pillage people for money (only tribes can do that). More money also means you can pay more bribes to get people to like you or join your plots. Also, if your treasury goes negative, you’re in for a world of hurt that can last longer than when you’re back in the black. Finally, to avoid going broke, keep a reserve of 100 gold at all times. That emergency fund will save your life.

Second, you must keep your vassals happy and get rid of the ones who don’t. You can use bribes and honorary titles (found under the Council tab) to make people like you, and you have five choices for getting rid of vassals who don’t:

• Plot to have them killed. You’ll need at least 100% plot power for the event to fire and you run the risk of being discovered. The higher your plot power, the more likely the event is to fire.
  
• Plot to revoke their titles. This is, I think, the best way. You’ll only need 80% plot power and 1 backer to do it. Most of the time, the vassal will take up arms against you, but as long as they’re weak and have no allies, they’re easily beatable and you can let them rot in your dungeon. Not to mention you also gain control of their county or duchy with no tyranny like you would if you revoked their title outright.
  
• Provoke them into rebelling. How do you make people hate you? Tax them (but everyone will like you less for that) or give them the title of Court Jester - it makes them like you less and lose prestige. Or you can raise their levies and just wait. The longer you raise a vassal levy for, the more they'll hate you.
  
• If they’re part of a plot (any plot), imprison them. Your chance of successfully imprisoning them is affected by your and their Intrigue stats. If you fail in imprisoning them, they will revolt against you, which is great because now they’re traitors. Treason is grounds for revoking a title without penalty once you’ve beaten them. You cannot revoke titles for merely being part of a plot - you must attempt to imprison them first.
  
• Fabricate a claim on their County and revoke your vassal's title after you've done so for no tyranny penalty. This is hilarious but it can also provoke rebellions and I prefer to have my Chancellor fabricating claims abroad.

One more thing to consider regarding your vassals is to keep them as weak as possible. Don’t let any one vassal have more than one county or duchy title if you can help it. Also, family members, shy, kind and content characters make good vassals; ambitious and envious characters are bad vassals (avoid creating ambitious vassals at all costs!). If a vassal is starting to get a little too uppity, you must deal with them before they can become a threat.

Third, you need to keep an eye on what’s happening in Wales, England and Scotland, as we’ll be expanding there once we finish with Ireland. The less united they are, the better. Sometimes England will break down into warring duchies, and I’ve never seen Wales unite by itself. Scotland is a tougher nut to crack than it looks - all stuff we’ll get into later.

Fourth, consolidate your rule in Ireland by scheming to revoke counties and strategically not giving out titles that you’ve usurped or revoked from problem vassals. What I like to do is control all of the now-duchy of Mumu and the Duchy of Leinister outright (I hold all of five county and both duchy titles) and also the county of Dublin (it’s very, very rich). This will give you a solid power base with levies and tax income. If your demense (land you control directly) size allows it, control as many counties as you can.

Fifth, at least once a month, look at your Factions tab. If any one faction reaches 40% of your strength, you MUST deal with them. Use any of the methods for dealing with problem vassals listed above. You’ll get a warning when a faction reaches 80% of your strength, but you need to do what you can to prevent that from happening at all. You’re the King after all, no one should be challenging your right to rule!

Sixth, unless you want to learn how to manage Gavelkind (check out my guide on Gavelkind succession for that!) you should consider swtiching your Succession Laws. Gavelkind lets you hold more counties in your demense but succession can break your kingdom. Primogeniture now requires High Crown Authority to enact since the Horse Lords patch. Since it's highly unlikely that you've gotten your Crown Authority up that high yet, you need to consider the other options available to you:

• Elective Monarchy
  Under Elective Monarchy, anyone from your dynasty OR Dukes that are not from your dynasty can inherit. Elective Monarchy is good because it will keep your titles together and as long as you keep your vassals happy, you can effectively appoint your own successor. But that's only if your vassals like you! I'd also recommend waiting until your ruler is aged 50 or more to choose your preferred successor, because you will take opinion penalties if you switch it around multiple times.
  
  
• Tanistry
  Tanistry is unique to Celtic (Scottish, Irish, Welsh, Breton) characters and is a modified form of Elective Monarchy. Under Tanistry, your Duke and above level vassals vote on an heir from your dynasty only, meaning your family will never lose that top title in succession. However, under Tanistry, your vassals will tend to prefer older characters NOT of your immediate family (Say, you uncle or cousins over your sons).
  
  
• Ultimogeniture
  Wait, what the hell is this? Completely ahistorical (except for the Mongolians) and gamey as hell, Ultimogeniture succession is a diamond in the rough. It functions the opposite of Primogeniture - your YOUNGEST son will inherit all of your titles. So why on Earth would you want to do that? For starters, you don't need to worry about having a ton of children because you can just breed until you get an awesome heir. Second, you only need Low Crown Authority to use it. Third, What will end up happening is when your ruler dies, your heir will skew younger than if you had used any other sucession law. This means that your new ruler will have more opportunities to improve their stats and take advantage of the Long Reign opinion bonus eventually. Ultimogeniture has really grown on me as of late, but beware the long regency! If you have an infant/child poised to inherit, start stockpiling gold ASAP - you'll need it for bribes etc.
  
  Prioritize Stewardship, Diplomacy and Intrigue in that order when you choose your heir.

Now that you’ve finished conquering Ireland, consolidated your rule and secured powerful allies in Europe, it’s time to expand out of the emerald isle. Wales is conveniently close and weak, so we’ll go for that first.

Now for Wales. There are three ways you can gain land in Wales:

• Fabricate claims and press them
  
• Marry your sons to Welsh brides with inheritable claims (mouseover their claims on their character screen and you’ll see if they are or not) or marry your daughters to Welsh men with claims in matrilineal marriages, and press claims on behalf of your dynasts.
  
• Ask the Pope to excommunicate local Earls or Dukes and request to invade and depose them.

Strategy 1 works really well if you have a 20+ Chancellor, as they can fabricate claims quickly, and you’ll want to fabricate multiple claims so you can press them all at once. The reason for this is that while Wales will not be a kingdom, there are duchies created, and when you claim a county, you’re not declaring war on the count, but their liege lord, the Duke. For example, let’s say you’re eyeing a county in the Petty Kingdom/Duchy of Powys (in terms of game mechanics, Duchies and Petty Kingdoms are on the same level). There are four counties total in the de jure Duchy of Powys. So, when you claim one county in Powys, you’ll be fighting against the Duke for control of just that territory. But wait, what about those other 3 counties? Well, because you only pressed one claim, you’ll have to wait 10 years to try and get those because you’ll have a truce with the Duke of Powys, and breaking truces is baaaaad. But since you have an awesome Chancellor, if you can get claims on 2 or 3 counties in the Duchy of Powys, you’ll get much more out of your war against the Duke of Powys before you’re forced to be at peace with them. When you do this, make sure you select the "Press All Claims" option when you declare war. If you win 3 of the 4 counties, you can even usurp the title of Duke of Powys for yourself since you control more than half of its de jure counties.

Strategy 2 takes longer to play out but isn’t dependent on getting lucky with your Chancellor fabricating claims. The goal here, as eluded to previously, is to marry your dynasty members (indicated by the red blood drop on their portrait) to people in Wales with claims that their children can inherit. This way, you will have dynasty members that have pressable claims on territory (counties, duchies or even kingdoms sometimes), but there are restrictions to if and when these claims can be pressed (Ruler of desired territory is female, a regency or a child). Do NOT press claims on behalf of people that are NOT of your dynasty! They will not join your realm (UNLESS they are already landed AND are your vassal - both conditions must be true) and you'll have wasted time, money and troops. Dynasty members also tend to make better vassals in general because of the Same Dynasty opinion modifier.

Strategy 3 does work but is incredibly unreliable. You may convince one Pope to excommunicate a Welsh noble but then that Pope dies and the new one likes him more than you, and lifts the excommunication and your casus belli is no longer valid.

Use whichever means works to conquer 51% of Wales, and then create the Kingdom of Wales. Offer vassalization to everyone you didn’t conquer and conquer the ones who turn you down. To really get Wales under control, replace Welsh nobles with Irish nobles (by revoking or usurping their titles), and you’ll lose the Foreigner opinion penalty for your vassals in Wales, which will make it much easier to manage. Keep in mind that the larger your realm grows, the more internal threats you are going to have, so you must keep an eye on your factions tab.

Now you can rinse and repeat this process in England or Scotland. I find England to be easier because it tends to be disunited and thus you can grab entire duchies at a time with little resistance. Scotland is a tougher nut to crack than it looks because Scotland is usually in high favor of the Pope and goes on a lot of crusades, even though a united Kingdom of Scotland is weaker than a united Kingdom of England.

Before I finish this guide, I want to point out two potential complications, one minor and one major, that can make your life difficult. The minor complication is if your ruler gets excommunicated by a Pope who likes your rival more than you. You can pay a massive bribe to get your excommunication lifted, or you can use the intrigue event “Buy indulgence for my sins”, which also costs a pretty penny. Excommunication is bad because your vassals have legitimate cause to rebel and plot against you and other rulers have casus belli to invade and depose you. If you deal with it quickly enough, excommunication really isn’t that big of a deal.

The major complication I want to make you aware of is if the Holy Roman Empire makes its way into England. If this happens, there’s almost no way you’ll be able to create the Empire of Britannia, because after 100 years of being in a different Empire, England will no longer be de jure part of the Empire. Also, the strategy we’re going to use will likely result in the same thing happening to Ireland, since we’re playing a long game here. After you finish conquering Wales and you see that England is in the grip of the Holy Roman Empire, you can either work on the slog that is Scotland or you can:

• Make yourself a vassal of the Holy Roman Emperor
  
• Conquer England as you normally would, ideally one Duchy at a time
  
• Request duchies from the Holy Roman Emperor if he holds any England duchy titles (not likely, but possible)

The biggest issue you’ll run into here is that when the HRE changes its law to Medium Crown Authority, vassals of the HRE (including you) can no longer go to war with each other, which ends your game of conquest. To prevent this from happening, always join the “Lower Crown Authority” faction when it exists. This way there’s always pressure on the HRE to not increase it. You’ll also eventually likely want to declare independence from the HRE once you’ve finished gobbling up England, and you’ll have about half the manpower that the HRE has. Thus, you’ll need to make alliances with other major kingdoms in Europe. France is one, and you’ll have to look around to see who else has enough military might to help you stand up to the HRE. Or, you can start or join an Independence faction or bribe the Pope to excommunicate the Holy Roman Emperor, which will turn all of his vassals against him (and is pretty funny to see). You’ll need at least 50,000 troops to win your war if the emperor isn’t excommunicated.
At last, now you’re the ruler of Ireland, Wales, England and Scotland, and that brings my Ireland guide to an end! Good luck and Deus Vult!

Guide written in its entierty by myself, Satanic Doge.
Corrections provided by /u/Deimos56 and /u/Tuerai on /r/crusaderkings

Updates:
26 August 2015 - Added comment on changing succession laws to the "While You're Conquering" section

28 August 2015 - Corrected Strategy 2 in "Beyond Ireland" section regarding pressing claims of non-dynasts

4 September 2015 - Corrected succession stuff in "While You're Conquering..."

19 September 2015 - Completely overhauled succession discussion in "While You're Conqering..."

29 October 2015 - Minor edits to succession discission in "While You're Conquering..."

23 February 2018 - I've got an itch, and I'm going to see about getting this updated. Stay tuned....