We used to sing a version of "Polly Vaughn" with a local community choir. There is a pub in the town called "The Swan" and my wife and I reckoned the song was about the young man catching his fiancee in flagrante - "And he shot his own true love (sic) in a room at the Swan".

I seem to remember the Oyster Band did a good version of this as "Molly Bond".