North British Warrior

[Medicus matt]

But what of perhaps a concept of belong to a wider group that we now refer to as Celts?

Druids were said to meet annualy in central Gaul, whilst their most prominenet site may have been that little island off north Wales. This could be argued as implying a group identy spanning the lands in between.

And Catholic Bishops go to conferences in Rome every year.
That's just evidence of a pan-national religious identity, not a national one.