not sure if this is really a Celtic song, but it has a lot of Celtic instruments...
"Full Circle," by Loreena McKennitt
http://www.youtube.com/watch?v=RCOTUfR9d44 (http://www.youtube.com/watch?v=RCOTUfR9d44)
Ell, Loreena McKennitt is actually Canadian and this song is off of The Mask and The Mirror which featured Spanish, Celtic and Moroccan influence along with contributions taken from Yeats, Shakespeare and St. John of the Cross. One of her early albums was so influenced by Celtic music that it sounds very similar to Enya. But it is a beautiful song and I love her voice.