"HI folks- I'm feeling like this thread is a rather narrow "Celtic /Anglo" focus.."
Basically the whole discussion is based on comments made by Dave Harker, which were aimed at English song Julia
American, Australian and N.Z. former colonies would have been very much British based repertoires
Beyond that I personally know little, apart from the bits I picked up an International Ballad Conferences, which wery largely about the material rather than their origins and distribution - I would love to learn more
My father was a prisoner of war in Spain in the 1930s and he came back singing songs that were being made about that conflict, largely by soldiers - I sing a couple of them (in private) for my own pleasure   

"someone needs to set a limit of how far back we go"
As 'The Wedding of the the Frog and the Mouse' was mentioned in the 1500s as being "sung by shepherds" and was still being collected from source singers in the 20th century and Barbara Allen was an "old Scottish" lady in 1666, it's hard to draw a line on where to start
Some songs can be aged by their subject matter, but many have universal and timeless themes, and can't be
Jim Carroll