For the non-Scottish among you Lochnagar is a mountain not a loch, over 3000 feet high, near "royal" (Billy Connolly)Deeside. I've toiled often to its summit. The loch there is Loch Muick (pronounced Mick). Lord Byron's mother's family name was Gordon from this area so there was a good deal of the Scot in him. I'm not a big Corries fan but their version of this song is brilliant.
I don't have much of a musical ear but always suspected that Connolly/Locgnagar were the same tune, thanks for the confirmation. Being Irish, I reckon that the Scots stole it, just as they did "Loch Lomond" from "Red is the Rose" !!