The Barnards/Barnets and the Musgraves held lands all over the north in the 16th and 17th centuries. I seem to remember in another thread someone suggesting there were lots of places in the north of england called Musgrave. There's certainly one in North Yorkshire. Barnard Castle is also in North Yorkshire but it's much more likely the ballad was written by one of the usual suspects and he used a few well-known names of the period. The earliest versions are the early 17thc broadsides.