Just a thought !
I was reading a book by Karen Armstrong 'the history of God' in which she makes very strong and convincing argument of three main religions all originating from the one God, (Judaism, Christianity and Islam being the 3 religions). Her use of the word 'pagan' was to describe the religions which predated the 3 mainstream. Considering the 'pagan' religions continued long after the founding and seperation of the other 3, the term pagan would therefore be used to describe a religion which predates or non-conforms with the Jewish/Christian or Muslim beliefs.
Druidism is one such and a wealth of music in the Celtic 'Tradition' would have tremendous influences from pre-Christian times. Another book recently finished 'the Bodhran makers' by J.B.Keane mentioned the Catholic church's view of the traditional festivals and associated music being paganistic ( Wren dances )
As I say - just a thought with my tuppence worth !