Loreena McKennitt is a great recommendation. She has a way of turning any song into a great song. I think her choice of instruments is awesome and her voice is second to none. I would recommend Nights from the Alhambra, a 2 CD (and dvd if you desire to choose that edition) of an awesome concert with amazing musicians
Also anything by The Chieftains, The Long Black Veil is great. Be bold, try out a few things, Celtic is so wide.