It's a Scottish song but the tune is Irish - the Orange march "The Boyne Water" (a.k.a. "The Water of Boyne"), which is still sometimes played by Orange marching bands.

It has a vague resemblance to David Murphy's tune "Lord Mayo", which seems to date from shortly after the Battle of the Boyne.

If the Dubliners did PoR to a different major-key tune, I'd guess it was because they were ideologically motivated to avoid the original.