The Robert Louis Stevenson quotes mean little as the master of Ballantrae is from 1888. Well after the song became popular.
My own feelings are that it's a lovely, sentimental song written about the Napoleonics Wars. Going to France to seek one's fortune MAY be about the Wild Geese, but there are dozens of other, more likely possibilities. The Napoleonic Wars especially, as they were the most recent and vivid ones in people's imagination. Had been going on for generations. No need for historical convolutions like the Wild Geese, Jacobites or the Revolutionary War when the obvious stares you in the face.
Before anyone starts on about ordinary soldiers not carrying swords, take it as a bit of poetic licence.