There are finnfolk in my Wicked Lovely series.
I've not been to Eynhallow yet. I've done Roussay, Westray, South Ronaldsay, & Mainland Orkney. It's easy to see how the stories of selchies & finnfolk originated here when you spend a few weeks wandering. (It's also a lovely place, filled with archaeology, natural wonders, & serenity. I've spent a couple weeks there 3 out of the last 5 years.)
There is a useful resource that not only has Orcadian history, but also lore. The entry on finnfolk is http://www.orkneyjar.com/folklore/finfolk/index.html
Orkney can be argued to be the source of both the selchie & finnfolk stories. The chief distinction being that selchies are shifter-ish where finnfolk are amphibian but not shifters.
In a related note, over in the Hebrides (my other summer escape the past few years) you'll find a related type of fey creature: the Hebridean "Blue Men of the Minch" who are said to be in the waters near Lewis. They're sometimes called "storm kelpies." A neighbor there took me out to sea to look for whales, seals, & just roam. Again, the chop of the sea as the wildlife pop up and stare at you with very human faces makes this myth make perfect sense too.