we often get people saying 'Oh, I love that Irish music!' when we've been playing Welsh tunes all night...

Yes, we've had that happen here in Sussex. We were playing in the back room of a country pub while the locals were watching the wide screen telly in the front bar where England were getting comprehensively stuffed by the Irish in some sporting event or other. The cry went up "And they're playing *&@%£^ Irish music in the back" despite the fact that some of the tunes we were playing had been collected within a few miles of the pub.