Are you sure you mean medieval? Even the few ballads that are set in medieval times probably originated in the early modern period and even then we're only talking about a handful of ballads and not many more beyond 1550.

If the ballad indeed does relate to any real event then the most commonly accepted theory is it relates to an event that happened to the Laird of Craigton c1634 but it may well be older than that. However it is highly unlikely that the ballad itself is medieval. As for the story, well I suppose it's possible that could be much older. Certainly some of the stories told in the ballads can be traced back to medieval times, but not in ballad form in English.