Gosh, I'd have thought that the obvious song would've been mentioned by now!   It's An Mhaighdean Mhara, beautifully rendered by Altan on the album Island Angel (made shortly after the death of Frankie Kennedy, a tragedy of dreadful proportions). Not only is it a wonderful song, it's also a beautiful slow air, one which I think I learned from a Chieftains album many moons ago....