An Ireland-wank is pretty hard to do I gather; basically too peripheral with too little economic potential to make a strong Irish state plausible, let alone that state projecting major power eastward.
Still the British Isles are the strongest candidate.
I don't think a Celto-Roman Britain is all that implausible, so they demographically prevail and absorb what Saxon invaders make it there and survive. The question would be, would they have to be so Romanized to do so they'd be speaking a somewhat Celtic influenced Latin derivative like French, or could a British dialect prevail and become the vernacular of a British state (or constellation thereof) strong enough to survive the Germanic invasions
Such a dialect would surely still be heavily influenced by Latin and probably Germanic languages as well, since they'd face not only Saxons but eventually Nordic invaders.
Is there any basis for modern Scottish to not be essentially derived from Middle English as OTL but for the Celtic dialects to prevail, even if the demographics and royal power still reflect a lot of Saxon input? Or for Wales to retain a more durable independence and remain essentially Britannic, despite the presence of a strong English influence?
In Brittany of course a Celtic remnant survives OTL at least as strongly as in Wales or Ireland, but it is hard to see tipping the balance so that Brittany is in no way French, given their geographic position, unless a greater reservoir of Celtic background reinforces their independence from a British Isles base.
I think the formation of a strong post-Roman British state has got to be the key; it doesn't have to hold all of OTL England but it does have to hold onto key parts beyond OTL Wales, and come back strong enough so that it is the Germanized territories that are culturally and politically peripheral and the Celtic zone that is the heartland of the strongest realm on Great Britain. An early reconquest of the southern tier of the island and re-Celtification there is probably essential. Perhaps Germanized Britain is mainly then in the north; maybe in time the southern British realm assists Celtic Highlanders in remaining independent so there is an essentially Northumbrian/Viking middle zone (surely not called "England!") between a strong south British entity and a northern Celtic-Scottish periphery, with the British state including OTL Wales integrally but as OTL the economic and eventual political center of gravity being more around the lower Thames.
It might not be necessary for there to be a continual political regime representing the British state going back to the 5th century though that would be cool; perhaps some ATL-Normans eventually conquer it in full, but despite their strong Latinizing influence the language of the eventually resurgent British kingdom remains basically Celtic, albeit with all the bastardization OTL English enjoys.
It would be cool and romantic if such a south Britain were drawn into Irish affairs to the point that eventually Ireland as a whole was a more or less integral part of the British continuum, despite the distinctions between Gaelic and Brithonic ancestry and the geographical and other divisions. But I don't see what overcomes the essential division of the two islands compellingly; no doubt British, along with midland Germanic, adventurers will seek to carve out dynasties there but why should the Irish regard them as any less alien than English or Scots of OTL? (Perhaps no more alien to be sure, but lineages that get assimilated to Ireland would as OTL become more Irish that otherwise in their identity).
Still, if Ireland is doomed to become the dependent possession of the dominant power on Great Britain, at least if that dominant power is itself Celtic though not Gaelic, the new language that becomes dominant on the Emerald Isle would be another Celtic one, not a Germanic one.
So that's the greater part of two large and populous islands anyway, in a strategic location come the days of transAtlantic commerce too, and who is to say that a strong enough British realm might not eventually swallow up the whole archipelago, or anyway consolidate its rule over all Great Britain?
It would not be certain such an ATL Britannia would play the same role as the UK of OTL, but it seems likely enough it might, in which case continental sized regions of the rest of the world beyond Europe might wind up being largely Britannic speaking.