The problem with folk music generally is that the original sources cited for music generally comes from print and dated accordingly. This is not necessarily the definitive starting point for this music.

In some cases (a hell of a lot of them) it certainly is. Tunes like the "Irish" reel "The Musical Priest", which is a straightened-out version of a strathspey composed by William Marshall and published by him in 2 different keys and under 3 different titles about 100 years before we have any evidence that anyone in Ireland played it. For a dead cert that was written in 1780-1. If it had existed any earlier, one of Scotland's innumerable transcribers of any interesting tune going would have got hold of it. Those guys were *thorough*. And we know exactly how the sheet music market worked - it only functioned at all because the punters could count on the publishers to come out with new stuff. The selling point was always "here's something you haven't heard before". When somebody tried to palm off an old tune as something new, it wasn't long before they got picked up on it (none too politely).

We have a pretty good idea how the dance culture in Ireland developed, even for times and places where we have no actual tune or dance transcriptions. What would they have DONE with thousands of reels at a time when nobody danced to them and there were no instruments in wide use that could have played them? You might as well argue that the ancient Spanish Celts developed the printed circuit diagrams for modern microprocessors 400 years before anybody could make them.

There are lots of tunes known in Spanish tradition back to the Middle Ages, like the 200 Galician songs in the Cantigas de Santa Maria. Not a one of them bears any detectable resemblance to any tune from the British Isles. Comparative methodologies DO produce resemblances over far spans of time and space in other cultures, like Kodaly's comparison of Hungarian folk tunes with those of the Mari and other Uralic peoples in west-central Asia. The result of such efforts in the "Celtic" world is a big fat zero. There is no unified Celtic musical culture and never has been.