I'm new to the game, and am on my first playthrough where I've successfully become the King of Ireland. I've been eyeing up the title of King of Brittany, but I'm not sure of the best way to take it.
I've tried one plan, where I married my sister matrilineally to a courtier with a claim on the Duchy of Briezh (same territory). I've pressed the claim and won the war, so now the courtier is Duke, and my dynasty will take charge next generation. Although, I've realized this doesn't really help me - my nephew will become Duke of Briezh, vassal to the current King of Brittany, instead of me.
Even if I find someone with a claim on the Kingship itself, if I press his claim, he'll just become King of Brittany when I win, and still won't be part of Ireland. I suppose I'd have great relations with the Kingdom of Brittany, but I want that land for myself!
How can I take this second kingdom for myself and become King of Ireland and King of Brittany? Do I need to ask the Pope for some help? Marry into the royal family itself and eliminate the other heirs? Fabricate claims and take the kingdom county by county? Is there any way for me to get a claim on the kingship myself?