Christianity was already in Ireland by the time Patrick arrived. It wasn't, however, very popular. I know that Celtic and Roman Catholicism is different but I thought it was Patrick that provided the organizational (political) elements that he learned from his Roman upbringing. It then reverted to Celtic Catholicism at a later date. I could be wrong. This comes from a weak memory of the history of the dark ages.

I'll try to find a link that isn't Catholic.

Anyway, happy St Paddy's day. Only in America would people drink green beer and wear tiaras! Go figure...

Have fun!