Diz, it was your singing of this that made me think a bit about it so I was able to answer Lidy's query.
On the face of it it could be Irish or Scottish as the Stewarts travelled in both countries, and as Belle's own song ('The Berryfields of Blair', which doesn't seem to be in the DT) attests people came from all over to Blairgowrie. How much of the song is actually Belle's own composition, and how much of it came from the 'old ploughman' we'll probably never know for certain.