It occured to me that there are probably hundreds of songs that have been recorded by established folk singers/groups down the years that were originally written or recorded by completely non-folk singers/groups. I've listed some below and would be interested to see any others that people can think of.

Eliza Carthy - Wild Wood (Paul Weller)
Steeleye Span - Rag Doll (Four Seasons)
Steeleye Span - Peggy Sue (Buddy Holly)
Chris Wood - Out Come The Freaks (Was Not Was)
Martin Carthy - New York Mining Disaster 1941 (The Bee Gees)

You get the drift..............