I’d go with a mix of Irish/Scottish and Breton, maybe?
I am not from the British Isles, nor France, but my region has a strong celtic heritage, historically wise, they were originally celts, but romanisation became more prevalent here than in say those places you mention.
The language still keeps celtic words (beizo, billa, camba, carballo, cheda, bode, etc etc) and we have celtic customs and music, loads of stone, hills -not very pronounced-, green landscapes, loads of gorses…, and so on and so forth.
The hill-forts have very pronounced moats here, which provided a strong defense.
As unique unit, the romans called the princeps to the main military unit on those hill-forts, which formed centuries to fight.
However, that’s preroman history, that’s why yes, either Ireland or Scotland is the better choice for a medieval celt based civilization, among other reasons.
Imho, the important thing is making them unique. Like having druids or stuff like that, sacred places to perform rituals in the nature, etc.