Where, before the 19th C, did Brythons and Gaels share an identity besides 'we would like to make it clear that we're not those English bastards'? I'm curious.
Well, I'm no expert on the Breton culture (that's for sure), but the link is basically that both cultures see themselves descendants of the Celts who existed at the time of the Romans. That said, it's a very broad link; The Bretons, the Cornish and the Welsh are like one sub-group of Celts and the Irish and Scots make up the other.
You should understand something here, I'm not in favour of this idea of Celtic unity between Bretons and the Irish; as I said I consider us two sub-groups of a bigger cultural set. When people suggest Celtic empires made up of Scotland, Wales, Ireland, Cornwall and Brittany I tend to role my eyes. Historically, only a Irish-Scottish duel nation or Welsh controlled Cornwall are the only alternative nations were plausible without some major cultural change happening.
But does the fact that being a Gael was a very ethnically-defined identity necessarily mean that people accepted the modern and never ubiquitous notion of nation = state?
I think if you look back through Irish history there's near constant major attempts to form a state; I believe the old qoutable was the Irish attempted statehood "Six times within 300 years" between from the 16th Century onwards. Before the Tyrone's Rebellion of from 1594 to 1603 there was a major fragmented Ireland because we were under the thumb of the English, but if you go back before that domination, to before Strongbow's invasion, basically every single major Clan in Ireland was looking to unify the Island, much like Alfred the great did in England.
I thought it took until the Black Death? And yes, where most Normans became 'more Irish than the Irish themselves' (this varied with time though and the distinction between Gael and Hiberno-Norman lasted at least into the late 17th C), but the Norman arrival was the moment after which there was a permenant English stake in the island. Even a small region under actual English vassalage in practice gives them a stake in what goes on.
The Normans assimilated in the sense that they spoke Irish, observed Irish customs, intermarried with Irish families and generally behaved like their Irish brethren. That said, within the country Norman descended families were still referred to as "The Old English", whereas the original Irish clans were referred to as the "The Old Irish". The third group of people in the land were "The New English", who were the people who occupied "The Pale", the area in the east of the country that the English directly controlled and taxed. That's the type of thing that didn't die out til the 17th century, by at which point the "The Old English" had inter-married so much they just couldn't say they were in any way distinct from the Old Irish anymore.
Being Old English was more a prestige thing as I understand it. It was more about being able to say you're descended from nobility outside the small pool of noble clans in Ireland rather than being anything culturally different to the Irish.