From John Stickle, as Megan says; Pat Shuldham-Shaw recorded some 94 items from him over the years, both tunes and songs. He didn't publish The Unst Boat Song, though, as William Ratter had already noted it; it was first published by the Viking Society for Northern Research (as A Boat Song from Unst), but I don't have bibliographical details.

Is there any evidence that this is part of a longer song? Offhand, I don't find any translation or further background information; though both will be available somewhere, of course.

I thought that I was bad at geography; but confusing Unst and Uist? Good heavens. Our "Guest" must be from aways off.