This is the basic TL idea of mine that I've gleaned party from history, partly from here.
What if Rory O'Connor becomes king as OTL, but after a major victory over a Norman army, one that delays expansion by a few years? He uses his political capital to reform Irish law to allow for more centralization, but eventually his line does die out/lose power. Even though there is no High King in Eire anymore, the precedent and the law is still there. Centralization becomes possible, with the occasional chieftain claiming the throne, even though there is still squabbling and English expansion.
Jump to the campaign of Hugh O'Neill, where he is winning victory after victory
and gains real spanish support. He drives the English back into The Pale, and forces a punitive treaty.However, the English become more and more nasty as they are driven back, devastating the countryside to an extent the Irish begin to all realize just who the real enemy is. When peace comes, the chieftains begin to rebuild, Then they call a council to declare High King, and O'Neill is selected. Thus a dynasty is born, leading a divided, infighting, yet slowly centralizing Eire, knowing that an enemy is over the sea, wanting revenge. This could make for a few huge wars, and an ireland desperately trying to unite against a common enemy.