Well, I found the answer myself in an interview with the author. TL;DR: **the location of Thisby is deliberately ambiguous but intended to evoke the idea of Scotland or Ireland.**
> _That leads me to ask, is your island of Thisby in England? The setting felt vaguely North Atlantic but I couldn’t decide for certain._
>
> Thisby doesn’t exist but I was going for quasi-Irish or Scottish. I wanted the reader to bring their own ideas about island culture to it so I left it deliberately vague. My editor [David Levithan] was sure it was off the coast of Maine, but I think that was because he had just been to Maine.
It's worth noting that not all aspects of the culture of Thisby are inspired by Scotland or Ireland. The annual event of the Scorpio Races quite possibly takes inspiration from the real-life (but much less bloody) annual event of the Chincoteague Island Pony Swim, featured in the award-winning novel _Misty of Chincoteague_ which Maggie Stiefvater devoured as a child.