Every tune with "selkie" or seal associations I know of is printed in the appendix to David Thomson's book "The People of the Sea". Most have no real words (maybe a nonsense phrase). Not many of them have been recorded.

This one is neither slow nor for the fiddle:

Fishermans Song For Attracting Seals

but it's a really nice tune anyway. (It works well done as a free canon, one player doing the tune straight and another repeating fragments of it).