As far as the notion of a canon, I see clearly that it is a term with little value for discussing folk music.

In some situations it's a useful idea. Bartok made an interesting comparison between the tune repertoires he found in Romania and Slovakia: they had about the same number of melodies, but whereas in Romania every village had only a small repertoire of melodies, differing greatly from one village to the next, in Slovakia every practicing musician from every part of the country knew the whole of the Slovak repertoire. Presumably Slovakia was a much more mobile society and had had time to develop a unified canon.

What I see happening with the Irish tune session scene is something along the Slovak model - it's no great feat to memorize a new tune, so the Irish session repertoire worldwide is fairly uniform and getting more so thanks to modern methods of music distribution. Whereas the "acoustic"/"Americana" singaround scene in the area I live in (Midlothian, southern Scotland) is more like Romania - singers have very small repertoires, and recycle their (usually quite polished) favourites a lot, but there might not be much commonality with a similar scene 20 miles away. What drives that is simply that it takes so much more effort to get something like a verbally complicated John Prine song into a performable state than it does for a reel off a Sharon Shannon CD.