How can I easily consolidate dynasty holdings under one ruler?

Question

I'm following several guides' suggestions for learning the feudal system and started a game as a Count of Ireland. I've made it to the 3rd generation of Kings of Ireland and somehow managed to land a dynasty member on the Throne of England.

I think I did this to secure an alliance or claim on some English or Welsh duchy. I honestly don't remember how far back it goes or how they won the throne.
Edit: Dynasty viewer reveals that King^-1 married a niece matrilinearly to a Scottish Duke, who I remember granting land to and pressing a claim for. Turns out she was the only child to a mother who was a female Norman usurper I had married patrilinearly to my dynasty, so her son inherited the Kingdom of England. Quite a bit of politicking, if I had planned it :-) That's also how I lost the Scottish Duchy to Brittish control.

The problem is, they're being way more aggressive that previous Kings of England. They've taken several of my Welsh counties, several Irish counties and one Duchy I've had an eye on, and the Kingdom of Normandy (I think? It's called Bretton Breagh or something equally confusing).

So a thought occurred to me: since we're of the same dynasty, I should be able to engineer a way to get both thrones under the same ruler (with myself pulling the strings, naturally). What are some easy ways to accomplish that? Intermarrying? Obscure succession rules?

Is there a limit to types of realms you can consolidate with these tricks? I've read the Advanced Marriage Guide, but it's a little above my play level still so I wanted to get the specifics spelled out.

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Edit: Answering questions and points raised in other answers, and refining my question.

• Kingdoms I'm trying to consolidate are Ireland (mine) and England - apparently now called Lloegyr, ruled by a distant kinsman.

• The King of England is also King of Bretton Breagh - it changed names again, now called Brythoniaid, which consists of most of Wales, some of Southern England, a Southern Scottish Duchy, and most of Northern France, but not Brittany.

  Clicking 'De Jure' on the title screen reveals that this is in fact Wales.

• The King of Britain is younger than me, there are several older male members of the dynasty. My Irish King is listed as the head of the dynasty. Does that count for anything?

• Bonus question: why the name changes for the kingdoms? Does it change by the culture of the ruler? Is there a key compiled somewhere or do you just have to get good at recognizing coats of arms or toggling 'de jure' a bunch of times?

• Scotland still has a king, but control of its lands is fractured at the moment, about 40% Irish, 30% British, 20% Scottish and 10% Independent, so just ignore it for the purposes of this question.

• England has AC-Gavelkind succession.
• Wales has AC-Primogeniture succession.
• Ireland has Primogeniture succession.

• My current Irish King has not changed a succession law. Since I'm Irish, I can go Elective Monarchy or Tanistry to get direct control over heirs.

  Tanistry implies I can nominate anyone of the dynasty. Would that work if I could get enough people to vote for the Irish King of Britain? If I succeed at this, would I be able to use Tanistry for the other Kingdom titles? What if I formed the Empire of Britannia?

  Would Elective Monarchy would require candidates to have a claim on the title?

  Presumably I could change the inheritance laws of the other Kingdoms, if I do successfully inherit, right? Would that cause any more upheaval? I'm not really a foreigner, so hopefully it'd be no worse than the normal -50 worst case.

• Since we're of the same dynasty, the AI doesn't seem to object to a matrilinear marriage. But it does list Political Concerns: ----, so I can't arrange any marriages. Why is that? Does the AI King need more than one child before he'll risk giving a claim to a foreign ruler?

• Our kids are 2nd or 3rd cousins by now, is that enough to dodge inbreeding?

• Craig, thanks for bringing up the vassalization option. I still wasn't sure if that would work or not. In order to pull it off, one of us would have to form the Empire of Brittania and vassalize the other, right? Deliberately losing territory to England would eventually accomplish this, but I have a hunch it would be long and painful, with no guarantee the AI would preserve the dynastic control.

Sorry that's so many questions, but hopefully they're closely related enough to succession to fit into one reasonable answer.

Answer 1

The biggest thing working in your favour here would be adopting seniority succession, where the oldest dynasty member inherits your titles. With that, you stand a chance of having the King of England being the oldest dynasty member on your death. If that happens, then you take over the KofE as your player character, who also inherits the Kingdom of Ireland (etc), thus uniting the realms in a personal union. (From there, you can take a stab at Emperor of Britannia.) However, this is not guaranteed by any means; unless England adopts Seniority of their own accord, you have to be pretty lucky and/or murder-y to get the KofE to be the oldest dynasty member and then die before he does.

Aside from that, there's no mechanic I know of that's specific to merging realms shared within a dynasty. You're left with all the usual options that apply to non-dynasty realms: marriage/inheritance, vassalization, conquest. (Vassalization in either direction won't work for you since you're both kings.)

There's a couple of advantages you get from sharing a dynasty with your target ream ruler:

• You get a +5 opinion bonus among members of your dynasty, including the King of England. That's not a lot, but it will help you make political maneuvers.

• You'll also get extra dynasty prestige from having a dynasty member sitting on a large & important throne. That translates to some extra prestige for children born into the dynasty. High prestige means higher opinion boosts and better marriage options.

Both of these help, but neither are terribly significant, and both are usually dwarfed by the other factors that are specific to the characters in play at the time.

Conversely, there's two reasons why having a dynasty member as ruler of a target real makes the realm harder to absorb:

• If you're close kin (sharing a grandparent or closer), you run the risk of inbreeding; this makes absorption through marriage riskier.

• Dynasty members are always allies. Declaring war on an ally means that you must pay 50 prestige. If you don't have 50, you can't declare war (probably not a problem for a King).

Answer 2

I did some experiments with a save game and found the following:

• Tanistry allows any dynasty members to be voted for, even children and unlanded characters residing outside the realm. As such, I could nominate the King of England as my heir. However, no one else wanted to vote for him at the time.

  This could work to consolidate the titles, but it seems even more chancy than seniority, as the voting mechanism ruins a deterministic line of succession, so murdering presumptive nominees does not guarantee your candidate gets up next.

• Elective Monarchy has stricter requirements for eligible nominees. I could only nominate my Dukes, children and siblings. No outside my realm and dynasty had claims on the title, so I could not test claimant eligibility.

I could not test Seniority, as my stupid childhood regent granted a faction's demand to lower crown authority and I can't change a crown law until a succession occurs.

• As mentioned in the question, my kinsman King won't accept any marriages for 'Political concerns', so I can't test that yet either.