Well it doesn't surprise me that Andy M. Stewart gets mentioned so much on this topic! Off the top of my head I thought of "Bridget O'Malley". However, my overall vote goes to the song "Kilkelly, Ireland". When my group used to perform this at the Bristol Faire (albeit a slightly modified version) it was sure to catch a few wet eyes. It has the added attraction of being mostly made up of minor chords, which as Nigel Tufnel said are "the saddest chords".