Hi Jim,
The St Clair (Mansfield) version is almost word for word the version in my 1763 edition of Ramsay. How early is Ramsay's earlier printing of it? St Clair would have been very familiar with Ramsay being part of Edinburgh society.

Whether her Cassillis version predates Burns would be hard to say with any certainty. Her probable date of writing this down is c1775 but it could be 10 years either side.

St Clair has an extra stanza which looks like a commonplace from other ballads.