What was his dating? Do you recall if reconciles the Celts not being Christians, at least until the time of Christ of course? Some of the earliest Christians were in fact Celts, those in Galatia (Turkey) and other near areas to whom the Apostles were sent.
IIRC,the people he was talking about were from somewhere in France,and were pushed out by the Romans. They the settled in the British Isles,but gradually were pushed out of the Isles proper by population pressure and wars. They wound up in the Faeroe Islands and Orkney Islands,and lived mostly be trading walrus Ivory and other high value items like that,as well as fishing.
They were gradually pushed to the west by the Vikings,and were finally absorbed by the Esquimo tribes. Sorry-can't recall any dates as to their adption of Christianity.