Here's a confusing one - 'Dirty Old Town'

*not a folk song and written as part of a play about Salford

* became accepted by many as an Irish folk song after being recorded by the Dubliners

* became a pop song when recorded by the Pogues who used the term "gasworks wall" rather than the original "gasworks croft" presumably to make the phrase nearly rhyme when there are no other rhymes in the song (or because the Pogues had failed to do any research)

* "gasworks wall" now used widely in folk clubs, sessions and open mics

So a non-folk song goes through the folk process by becoming a pop song!