I too read "Celtic Christianity' by Joyce awhile back. A good read, but like you said it mixes in some of what sounds like New Age stuff.
I'll tell you how I feel, tho, about Christianity and the Celts... other reading has pointed out that the Celts & peoples of Ireland, England, Scotland were perhaps pagan but when Christianity arrived on their shores they were some of the most enthusiastic embracers of the faith. Sincere, true, and devoted. You've read about St. Patrick, St. Brigid, etc., ah they are such inspirations to me even now.
Personally as my own faith in Christ has grown and matured over the years, I've come to see that He's a lot bigger than the denomination I belong to... He's bigger, all encompassing, and He makes himself known to all peoples in a variety of ways. Evidence of Him is all around us, in the trees and stars and nature - it's His handiwork! Glorious. Some people worship nature, and now I just see it as a bit misguided but well-intentioned... I always hope that thru an appreciation of nature that some day they will be faced with His true presence.
"All that is gold does not glitter, Not all those who wander are lost;
The old that is strong does not wither, Deep roots are not reached by the frost.
From the ashes a fire shall be woken, A light from the shadows shall spring;
Renewed shall be blade that was broken, The crownless again shall be king..."SGGardner Art