The fact that you can sing the words of one tune to the melody of another simply means that the tunes are in the same metre - there doesn't have to be any melodic connection between the tunes.

Here's a link with some information (and some further speculation) abut the origins of the song:

http://appalachianlifestyles.blogspot.com/2009/03/in-search-of-rose-conelly.html

Marje