There have been some great threads on selkie songs, one of the best-known being the Selkie from Sulskery (as MMArio points out, you need to try a lot of alternate spellings, including selchie).

Animaterra, I'm jealous you met Mary McLaughlin, Yunda is one of my favorite tracks off a celtic women compilation CD. You might try searching some of the celtic language sites for clues to the source.