Random note. One of my favorite instruments is the wire-strung Celtic harp. It was killed by several things. First of all it didn't include the sharps and the flats. Secondly it has a 10-30 second sustain. Because of those two factors it sounds better in the older tunings. Because of those two factors it sounds horrible playing more complex modern music.
Also in the Celtic vein: uilleann pipes. Anything played in the key of D with drones sounds fantastic, since it is tuned using just temperament: http://www.youtube.com/watch?v=FW0foorxhDk
A lot of the old Irish fiddle music was actually originally intended for the harp. If you want to know what it sounds like, I highly recommend Patrick Ball. http://www.amazon.com/Celtic-Harp-Vol-Turlough-OCarolan/dp/B...
(I don't know what tuning he is using. I strongly suspect equal temperment.)