Belloc, I had a look at the linked dictionary, & must warn you that it appears to be written by infidels, they use the Judaic C.E., rather than A.D. in the articles in their encyclopaedia, also there are many articles on druidic feast days, ancient pagan gods & so forth, but none on the Church, St. Patrick, St. Columba, St. David &c.
Continuing on the subject of the Celts, I find it interesting that many other European countries have a history of Celtic habitation. There were Celts in Portugal, Spain, northern Italy, Bavaria, Austria, Czechoslovakia & even Turkey, in addition to the more well known countries. I would imagine therefore, that many Bavarians, Portuguese &c., have some Celtic blood.