I have never really heard or read much about the Celtic tradition of faith, and honestly this man doesn't exactly promote it. But poet and author John O'Donohue is what I think of as a modern day mystic and for that, I like to think of him as Celtic. He is after all a native Irish man.
I ran across him listening to the Speaking of Faith podcast. It's an NPR show that airs here on Sunday mornings. The podcast is entitled "The Inner Landscape of Beauty." I think I have listened to it now 5 times. It is the sort of thing that I just can't get enough of. A conversation about a faith integrated with a longing for beauty and an intense relationship to God and a friends. I highly recommend downloading it.
Although he passed away this January, his newest book of blessings has just been released. It is To Bless the Space Between Us: A book of Blessings. I've purchased it and his beautifully entitled book Anam Cara: A Book of Celtic Wisdom. Anam Cara means Soul Friend. You can see why I was immediately attracted to it.