Someone will probably sing "Autumn Leaves" tonight: that's not folk, unless...maybe a nice wee finger-picking accompaniment??
Pachelbel's Canon - not folk but that chord progression re-appears in 100's of "folk songs".

And Trad Irish - Tom Paxton also has had his "Last Thing on my mind" given this accolade.
And Eric Bogle was pronounced dead long before his time, when Tony Blair said that his favourite song was "No Man's Land (Green Fields of France) - written by a First World War soldier", as Eric loved to tell us at his concerts!