The interplay between what gets sung in church and what gets sung in social gatherings has always been a two-way process. I suspect there are as many tunes which started off as hymns as the other ways round.

As for an example of a song which has gone from folk to church and back again - I seem to remember hearing a rugby song derived from To be a Pilgrim, which of course uses the tune associated with The Blacksmith. (Though it wouldn't the least surprise me if someone finds out someday that it had previously been used for another hymn, and maybe was a song before that...)

An interesting short discussion of how the Sally Army used Music Hall tunes according to the principle alluded to in the thread title.