A lot of films made on Scottish and Irish themes have trad music performers in them, or providing tracks:

Brothers McMullen soundtrack was done by Seamus Egan and friends.

Capercaillie performs in the film 'Rob Roy' in addition to contributing to the soundtrack along with Davy Spillane, I think.

Bravehart had Chieftains didn't it?

And what about that Jodie Foster movie 'Nell'? Didn't that have trad music too?

I'll think of more later, I'm sure.