One thing i don't see metioned in the rather good explanation Ed has linked us with is that the common form used for these is a four line stanza commonly called a come-all-ye because so many of these songs started that way. The singer was free to adapt the song to whatever melody that fit, and they often did. That is why so many of these songs seem to be sung to different melodies. The immigrant or the rural transplant would have many of these to chose from.

Brían