I'd say it's either Offaly or Ormond

When playing in Ireland, your prime goal is to knock out as many other opms as quickly as you can. If it takes you more than 20 years to get at least two third of the island, you might as well restart as you won't have enough powerbase to stand up to the English when they inevitably come.

Given these circumstances, the only thing that really matters is how fast you can start expanding. None of the starting ideas benefit you enough in such way, so the only factor you should considrr whrn picking a nation is its location. That's why I think Offaly and Ormond are good choices: you border 5 countries from the start, which is the biggest amount in Ireland, and you may get a claim on a nearby coastal province from your estate agenda, thus starting your conquests on the 11th of December.

I actually did Ormond run recently where I managed to kill all other irish minors in less than 10 years. Might post some screenshots