This from Dick Gaughan's notes, as indicated in the link above:

The original text was an attack upon the Treaty of Union of 1707 which abolished the independent Scots and English Parliaments and set up the United Kingdom. I made some minor amendments to give it contemporary relevance.

The tune has been the subject of some speculation and argument. So far as I am aware, I actually composed it and am highly flattered by the presumption that it is traditional, with people claiming to have known it for several decades, if not centuries.