I play quite a bit of Irish music, I think the style of music is sort of split in two. The melody is driven along by the guitars and bodhrans (to name only a few) and as a result not much harmony is needed. Altan demonstrate this. Often instruments like citterns, bouzikis, mandolings can create other harmonmy likes while driving the intense melody. Donal Lunny is a fine example of this accompanymeny.

Cheers, Sam