I think talking about "the original folk song" to which another song was written is to misinterpret the process by which (folk)songs grow and develop. You can look for the earliest version, and for earlier versions of related song, but looking for "the original" is like looking for the foot of a rainbow.

Even when so eone sits down and makes up a song and makes up a tune for it, if its any good, when you dig hard enough you can always find another song and another tune that fed into it.