I think Bruce makes an important, and often overlooked point,(if I'd have thought of it, it would have been easier to write my first response) the melodies to folk songs were predominantly sung unaccompanied--the guitar accompaniment(and the four part vocal harmonies, at least as we know them) are a fairly recent, and generally "commercial" addition--Until the time of the field recording, most of the folk music that was collected tended to have been "evened out" following the rules of common pratice classical music by the people who transcribed it, sometimes just because they had no good way of notating what they heard, sometimes because they believed what they heard was "wrong", and sometimes because they just didn't hear what was being sung--

For chordal acompaniments, the commercial arrangers simply create an accompaniment that conforms to the sound of whatever is popular--Leaving us with things like Glenn Miller's swing version of Little Brown Jug and Nirvana's grunge version of In the Pines--Either one of them could have done the same to Sweet Molly Malone, leaving us with chords that probably wouldn't reflect much of the melody--