Edith Fowke, "Lumbering Songs from the Northern Woods" (AFS, 1970), p 7:

"... many of the singers had the typical Irish habit of speaking the last word or phrase of a song (indicated by italics in the song texts).

"The practice of speaking the last words is so characteristic of the lumbercamp singers that some folklorists have assumed it originated with them, but actually it turns up wherever Irish traditions predominate. Elisabeth Greenleaf notes that it was 'a perfectly familiar convention to a Newfoundland audience' ... "

There's a couple of paragraphs further, with reference to the Miramichi, to Australia, and to some 20th century recordings including the Caedmon anthologies. She quotes a folklorist who supposes it a British tradition that had died out at home, but Fowke urges that it is Irish, rather than British.

Of Fowke's informants, Emerson Woodcock was very prone to speaking the last phrase or word of a song, though -- like O J Abbott -- he doesn't do it on evert song in his repertoire. Similarly, such Irish trad singers as John Doherty, Thomas Moran, John Maguire, and at least some of the performers on "Folk Ballads of Donegal and Derry" also use the convention. I don't have any recordings of singers from US lumber camps of the upper Midwest but would not be surprised to find that older informants there also speak part of the last line (it's never the whole last line, just the last phrase or sometimes the last word, or even syllable -- Fowke notes one in which "Michi-" is sung and "gan" spoken). I've never heard a singer do this in the Hillbilly Belt but it may well occur in the northeastern US, abutting Canada.