Well Richard one of them is the Mudcat itself ;-)

a few others are;

http://www.sosyourmom.com/Oneils1.html
(O'neill's music of ireland DB)
http://www.geocities.com/SouthBeach/Marina/4870/midi.html
(irish and "celtic" midis)
and
http://www.traditionalmusic.co.uk
which has very large collections of trad from all over the world