In The Wee Free Men - and other places where the Nac Mac Feegle are mentioned - they are quite obviously depicted as Scottish. The easiest example of that is their speech:
"Crivens! We wanna coo beastie, no' a ship beastie!"
"Ach, one's as goo' as t'other! C'mon, lads, a' grab aholt o' a leg!"
"Aye, all the coos are inna shed, we tak what we can!"
The Wee Free Men, chapter 3: "Hunt the Hag"
They also wear kilts, are red-haired, and get drunk a lot - all of which are associated with the Scots.
Is there any particular reason why the Nac Mag Feegle are Scottish? (In-universe, it doesn't look like anyone else speaks or behaves like them (including the Fairy Queen (or the "Quin")), but I haven't read all of the books yet.) Out of universe, what purpose does them being Scottish serve for the story?