I read that the Black Death shattered English authority in Ireland for the rest of the 14th and 15th Centuries, reducing them to a small stretch of land called "The Pale" around Dublin. So, WI some Irish tribes launched an offensive against the English and drove them off the Emerald Isle completely? As in no more English political strenth on the island, at least for a few decades or a century?
Hm, I'd always heard it was due to England focusing so much time, effort, men and money on France, but both are likely true.
I could see some chieftans allying to expel the Sassenach, but what happens after is the interesting part. Do they revert to the old 'every clan for itself' mentality, or could a successful war lead to a proto-nationalism, as it did in France and Scotland?
You'd likely need a Great Man(tm) to forge the feuding Ri into a reasonably unified state. Could also use some French help, keep the English surrounded on all sides, that sort of thing. Hm, that might actually be an interesting idea.