hey pattyClink--I'll support your foundationless theory! I do think there are different "traditional" ways of harmonizing which are pretty old, actually. It is just that from where we sit, that type of singing doesn't fit neatly in the academic "definition of folk" (sic). Part of the problem is that so-called folk music scholars of Britain, Ireland, and the US haven't recognized and included women's traditional singing in the definition.
There isn't supposed to be call and response in Irish singing either, but if you include what women actually sing, and sang historically, call and response is part and parcel of the tradition, just as unison singing is part and parcel of the tradition.
Noreen--Lillis O Laoire has done some interesting research on the unison singing of women in Donegal, and Angela Bourke has done quite a bit of research into the women's tradition of caoineadh, or keening. Female lament, like female work songs (waulking songs) in Scotland, both use call and response, and unison singing of choruses. It is my foundationless theory that when the priesthood effectively prohibited the performance of female laments in Ireland, that women's vocal energy was put into church music, where harmony and unison singing was more the norm.