It's not entirely by Tannahill. A shorter version of the words was printed by John Hamilton in 1792, and the tune was published by Bremner in the 1750s, so Hamilton himself must have been adapting a traditional song which hasn't come down to us. Bruce Olson posted my copy of Hamilton's version in another thread on this song a few years ago.

Tannahill was fond of fantasizing about places he'd never been to. Check out "The Banks of Spey" for an example (that one he *did* write, to fit William Marshall's tune of the same name). He makes north-east Scotland sound like Svalbard.