I think that if you go into the startmap files you will see that the Irish are Catholic Isolationist. I am not sure how much this affects their chances of expansion, but I have never see them try and only once I saw them building a single sheep. When controlled by the AI their usual role is to get the English or sometime the Danes excommunicated (same as Scotland's role). I have also noticed that the irish rarely accept an alliance proposal, even when you offer them a princess and they have 5 unmarried heirs.