Years ago I heard a very sour Pole or Hungarian on a bus in Dublin mocking Irish music, basically saying "They claim all these dance tunes, but they're all ours!" My impression is that people travel for work and after work everyone takes out the fiddles and pips and whistles and starts playing and dancing and going "Oooh, that's a nice tune, I'll 'ave that", and tunes, promiscuous creatures that they are, travel happily home with their new boy- or girlfriend and spread there.

The interpretation may be different - Scottish music has, for me, a pleasantly lemony flavour different from Irish music; klezmer tunes have their own wildness; French country music has an almost Hungarian sound - but the same tunes are flying around all over the place.