I suppose it's a question of whether you see the Roman Church as separate from the Roman State. As was mentioned here earlier, the Roman State didn't really care what language its subjects spoke as long as they paid their taxes on time. The Roman Church was bent on making converts, and part of that prorgram includes language: bible translations, coinage of new words for foreign concepts, loanwords. The Church did not step into the vacuum created by the departing legions, they came with the legions.

As for the Welsh being the Irish who couldn't swim, Maverick, my understanding is that the Welsh are the descendents of the Celts who occupied most of today's England, while the Irish are the descendents of the Celts who occupied Ireland. In a sense they both had to be good swimmers to get to where they are today.