*waves Irish flag*
With a POD before the Roman invasion of Britain (or shortly thereafter), how could it be possible to get the British Isles united under the flag of Eire instead of the other way around?
I'm not a scholar of British history (though I do have an incredibly-detailed volume called "The History of Britain" that goes as far back as the Bronze-Age mound builders there), so I'm not going to try and make suppositions about the setup.
But, IIRC, a large reason why Britain was able to develop as quickly into a feudal system of feuding Kings was because of the constant invasions. But the Vikings settled in Ireland as quickly and voraciously as they raided Britain. Understanding that, would there be any way to, say, have a Norman or Saxon defeat in Britain, but later on realize a victory in Ireland, causing a similar development of society, government, etc. that would allow them at least a fighting chance at an invasion of Britain?
It's just something that bugs me sometimes when my inner IRA member screams for the unification of Ireland.