Mary and the original poster mentioned Welsh songs - and Mary gave the words to one of the prettiest and perhaps best known. There are actually quite a few, and many of them 'fit' a lot of the theory above.

A Ei Di Deryn Du (Blackbird, will you go?) as posted by Mary

Bachgen Bach o Dincar - which was discussed in another thread some time ago, I think in connection with some Australian songs

Y Ferch o Fedlam (The Girl from Bedlam) - a variation of Dives and Lazarus

Ar y Ffordd wrth Fynd i Rymni (On the Road to Rhymni) - which I always tended to avoid in public singing due to the repeating line, "Very Well done Jim Crow". I recently found out that, in the area from which it came, there's a historic cottage known to everyone as "Jim Crow Cottage" so this may require some further research ...

Dingl Ding Joseph - a children's song

A number of ballads from the 18th C, particularly those of Jac Glan Gors

Can Merthyr, which is the only one that I have to memory, first verse being:

Ye lads all through the country
Gwrandewch (listen) unto my story:
You'd better go dros Ben y Graig (over Pen y Graig)
Than go with gwraig (wife) to gwely (bed).

The singer goes on to complain about his wife who gives him thin soup and no tobacco.

siân