Well, I dont know about your collections, Orac, but I reckon the Ossian books outsell everything else ten to one, judging by the fact you find them in every shop, and everyone I know seems to have a few of them. And they are definitely labelled "Folksongs and Ballads popular in Ireland" (admittedly with the "popular in " in rather smaller print). Every one of the songs you quote are in the ossian books,incidentally.
And you'll have to better than just say "quite clearly" to convince me Danny Boy was written about Scotland. That's just an assertion: what's your evidence, or reasoning? Maybe it was, I'm just curious to know. But in general I will say I define folksongs in terms of usage, not origin, once they have bedded in and become part of the landscape.