When the witches became Christians in Ireland, they carried over quite a lot of their traditions and art -- and Christianized them. This was also their way of making Christianity seeker-sensitive. Then when civilization came out of Ireland at the end of the dark ages these Christianized traditions came with it.
The Christians brought over numerous festivities simultaneous to the witch festivities but with different names.
There were a lot of beautiful tunes that were brought over by witch musicians who became Christian. The Ash Grove is a little obvious example.