The fun lies in the way a performer can sometimes put across a non-folk song in a style that makes people hear it as a folk song. Or the other way round with a cross-dressed folk song.

Either way it needs to be a good song.

(The reason I mentioned "Tie a Yellow Ribbon" lay in the previous post by Steamin' Willie - If I were to sing "Tie a yellow Ribbon Round the Old Oak tree" then ergo it is a folk song. True enough many worse songs. Many even worse songs.)