Sandy:

I neglected to mention that a search of Digitrad would also turn up a number of threads discussing Loch Lomond.   A particularly interesting one is    here.    It is of course not unlikely that Lady Scott re-worked an existing song (not necessarily Jacobite), just as Burns re-wrote songs that he picked up, making them more "polished" (i.e. "literary").

You can also find an arranged version of the tune -in midi format- at Lesley Nelson's site,   here.  

It would also be worth looking in your local Public Library, if you have one; it's probably not as old a song as a lot of people think, but it is very widely known, and widely published.

Malcolm