I agree. "Instantly recognizable" is a misnomer as the people above have related. The scales and such that you mention in the original message can be found to be different in Scottish and probably Irish Gaelic songs which are known to be from earlier than the 18th century. Pentatonic scales and other scales of that sort were common in Gaelic songs and tunes.

If you break down "Celtic" into Gaelic and non-Gaelic, you might have something "recognizable" or close to it. At least there, you would probably only be dealing with Scotland, Ireland, Wales, Cornwall, Brittany, Man, and parts of Galacia, all of which are known to be "Celtic" countries.

Perhaps!