Starting with a POD no earlier than 410 AD, create a timeline where Ireland is a great power 1000 years later.
By great power, I mean something on par with OTL's Castille/Spain, France, Sweden/Kalmar Union, England, or the Ottoman Empire--a state that either already is (or is on the cusp of being) a major empire.
The only major condition is this: The ethnic identity has to be Irish, something like Ulster won't do. The nobility must be Irish-speaking. So no situations where an English dynasty has to flee to Ireland, takes over, and then Anglicizes the place. Nothing like that.
To my way of thinking (and probably everyone else's), this is a really hard thing to do, because Ireland is overshadowed by its loutish eastern neighbor. So just spitballing, I'm thinking Ireland has to unite soon after the fall of the Roman Empire in Britain, and somehow forge a lasting union with Dumnonia and the Welsh states. This doesn't seem totally outside the realm of plausibility, because Irish tribes actively raided and settled along the Welsh coast during this period. The trick seems to be getting them to hold the territory. A semi-united Cornwall, Wales, and Ireland would seem to be more than a match for any of the Romano British petty kingdoms, but I'm not an expert by any means.
What are your thoughts?