Most of the replies you've got miss the requirements you set out:

- traditional (which Ashcan Farewell isn't, neither is Da Slockit Light)

- not a song (so not The Great Selkie - the usual tune is still in copyright anyway)

- for the fiddle (Lonesome Boatman is specifically for the whistle, as well as not being traditional; most of Carolan's tunes work best on the harp)

- something to do with selkies, the sea or seals (that cuts things down a LOT).

Which of these factors actually matters for what you're trying to do?