Back in college, one of my roomates and I did a study on Celtic music.
One of the oddly strange things I found was that I found a bunch of great Celtic music in one of the least likely places (in my mind)- from the "Lifescapes" music display at Target. A lot of the music was fantastic, especially the instrumentals.
They have a wide variety of styles, though none are your modern blends (suchas punk, rock, etc, like Dropkick Murphys and Flogging Molly). You have some Folk, Bagpipes, Fawn, Jigs, guitar/lutes, flutes, and various combinations.
"Celtic Bagpipes" and "Celtic Mystery" were my favorite. "Celtic Mystery" has just over 72 min of celtic music on the album. Here's a link to 25 of the albums from there, and you can listen to samples of each album, of each song. Each song is $0.99 or $9.99 per album:
http://www.lifescapesmusic.com/CustomEditorial/celtic.aspx