"Celtic" I think we have established by now is meaningless when used to describe a kind of music. But that isn't to say that you can't hear different accents in music.

Imagine for example versions of the same tune- for the sake of argument, let's say "Soldier's Joy"- and hear it played by

- An Irish fiddler in the Coleman style
- An Irish fiddler in Donegal style
- An English dance band fiddler of the 60s generation
- A Scottish fiddler
- A Shetland fiddler
- A Northumbrian fiddler
- A Cape Breton fiddler
- An Old Time fiddler.... (the list goes on)

You would definitely be able to tell differences, though not always associate the single tune with its cultural origin(*). Over the course of, say, a record I think you would normally hear a trend develop (though this would often have to do with other clues like style of accompanyment) which would complete the picture.

A student could (and probably has) develop a taxonomy of the styles, indicating possible developmental links which could be explored further by studying migration and trade patterns etc.

I'll kick off with one possible linking influence- the fishing industry. The groups from the British Isles broadly fished the same waters, and could be expected to meet up socially on occasions. This would have disseminated tunes and instruments back into their communities, leaving them to develop in the local style.

(*) Someone did have a project to examine perceived associations, though I thought it a bit half- cocked as they used strange tunes. Anyway, I've lost the link.