"good evidence of them starting out as 'popular music' "
There is no real "good evidence" - just possibilities based on publising dates, but Walter Pardon sang a beautiful version of 'The Rambling Blade' which he once described as "the best old folk song ever written" - I can't find a recording on-line but he can be heard singing it on his early albums
The somg may have originated as a printed 'goodnight ballad', but nobody can say fro certain and it is possible that it came from a 'common source'
There are no certainties in any of this - just possibilities
Jim Carroll