"Robert Tannahill did not write a "Wee Drappie O't" as MacColl said."
I think we managed to trace the probable author of 'Drappie' from a hand written note in our set of 'Vagabond Songs of Scotland' once owned by Will Walker.
MacColl's error has, as you say, been taken up by many, but the fact that his family believed it back in the 1930s is indicative that it has been around for some time
Jim Carroll