If yer looking for a "folk song writer" perhaps a way of parsing it out is: who is the known author who has the most songs that other folks have thought was "Trad?"

Works of Robert Burns, Si Kahn, Pete Seeger, some of the Scottish and Irish writers in a traditional vein have had that misattribution. In some cases when newer songs (especially from the other side of the Atlantic) are performed by singers who also do a good line in the very old songs, that the "trad" gets assumed by listeners not aware of the actual author. I particularly got a kick out of being earnestly told that "The Scotsman" was a very traditional song, and couldn't possibly have been written by an American bluegrass guy...

Songwriters who have multiple works written in my lifetime that I believe will live on "in the tradition:"
Woody Guthrie
Pete Seeger
Eric Bogle
Gordon Bok
Phil Ochs
Sidney Carter
Tom Paxton
Andy M. Stewart
Tommy Sands