This classic was created in 1894 for the premiere of an operetta of the same name, based loosely on the life of Scottish folk hero Rob Roy MacGregor.
It's basically a Manhattan made with with Scotch.
The traditional garnish is two cherries pinned together, no doubt a reference to the romantic interest in the operetta.
This would never have been made with a fine single malt Scotch - Scotch whiskey was almost all blends in those days. I have good luck with the Famous Grouse.