There was a BBC radio show on Cash a year ago. I believe it said that John originally thought his family came from Ireland, and so may have been inspired to write 40 Shades after a visit there. But he later found out his roots were really in Scotland .
There's a nice version by Rodney Crowell that Frank Hennessy plays on Celtic heartbeat, Radio Wales. E-mail Frank and I'd bet he'd play it again, and Frank may even find this thread.

Chris