"almost celtic sounding but not quite". Well, I'd say that Finnish music sounds quite like Swedish music, Swedish music sounds quite like Norwegian music, Norwegian music sounds quie like Shetland music, Shetland music sounds quitelike northern Scottish music, northern Scottish music sounds quite like southern Scottish music, southern Scottish music sounds quite like northern English music, northern English music like southern English music blah blah blah. factor in a few sea-borne cultural short-cuts and you've got a pretty coherent picture.The simplest explanation is the best, unless you can find strong evidence to the contrary (Occam's razor).
Mythical 2000 year old history will never beat real 200 year old contacts.Musical links are made by real musicians doing real comunication as far as I can see! And what does Celtic music sound like? What exactly links Scottish and Galician, as opposed to Basque and Shetland, for example?