[Prudence]

It's hard to come up with a good source for "Celtic Christianity." There's not a lot of written documentation from that timeperiod from the perspective of the Celts on anything, including religion. It's a difficult area in which to pursue scholarly research. The popularity of modern Wicca and attempts to reconstruct Druidic ritual confuse the field farther and it's even more difficult to separate solid research from conjecture -- or even conjecture based on historical evidence from wildly hypothetical conjecture.

Thus, I've not spent any time in that area, except to note that there is periodic evidence of practices that don't conform to Rome. Much of it's from early in church development, when even continental practices weren't cohesive. As orthodox practices trickled west, they were eventually adopted on the island, too, it just took a little longer, perhaps. I believe that co-ed monasteries were approximately the same time as the continental church was still working through priestly celibacy issue.

It's an oversimplification, but I believe that "celtic churches" retained and incorporated local beliefs in much the same way as isolated continental villages did. (See "Night Battles" by Carlo Ginzburg for an example from Italy.) The British Isles, especially the outer reaches (Scotland, Wales, Ireland, Isles of Man, and so forth) were sufficiently geographically isolated that lots of things were different there over time. Heresies come late there and are more socio-political than doctrinal, for example.