As Rich-Joy suggests, the song was written (as Song To the Seals) by Harold Boulton (1859-1935) and Granville Bantock (1868-1946). It is not public domain and is still in copyright, though a good few performers have ignorantly passed it off as traditional. The pianist Stephen Hough, who is among many who have recorded it (with attribution), quotes from a printed score: "The refrain of this song was actually used recently on an Hebridean island by a singer who thereby attracted a quantity of seals to gather round and listen intently to the singing".

This is not evidence, incidentally, that the melody of the refrain was taken from tradition (though it's not impossible); there are always such stories. Marjorie Kennedy-Fraser attracted an audience of seals in the North Bay of Barra by singing The Seal-Woman's Sea Joy to them.