I finished my game of Ireland today in about 1220. Could I have continued? Sure. But, I just... couldn't. Losing ground is tough. But, I think I achieved everything I wanted there.
I formed the Kingdom of Ireland, sanctioned an invasion on England and took the crown. I had enough of Wales and Cornwall now to form my third crown, Brythain, and finally, one of my daughters matrilineally married to a Prince of Scotland, was now Queen of Scotland, and her heir was my grandson. Two assassinations later, I had all four crowns under my belt.
Unfortunately, I was pushing 65 years old and died soon after. My son was not the most ideal ruler, and combined with the overexpansion and foreign culture differences, I ended up having to pass the crowns out until all I was left with was Ireland. (11 vassals with 70%+ chances to rebel. Ouch.)
I may have lost, but, there was a big "UA RUAIRC" splattered across all of the Isles.
The Irish Ua Ruarics - My line.
The Welsh Ua Ruaircs - Dominating Brythain (sans Brittany, they were eaten by a half-unified Iberia under the Castille family)
The Saxon Ua Ruaircs - Every Dynasty member I could find without land was given duchies/counties after my invasion.
The Scottish/Norwegian Ua Ruairc - Descendants from my first ruler's second son, given The Isles as a pittance. Gave them the Scottish crown and let them deal with the revolts.
Obviously, my issue came up with my rulers converting to the local culture. >.>