The earliest forms of the tune are for a Scottish Gaelic song, "Mor nighean a Ghiberlain" (Morag the beggar's daughter) from the 18th century (printed and written in manuscripts with at least as many spellings as there were copies). It was also used around 1800 for a song by Tannahill, "My Braw Highland Laddie", which survives as a ceilidh-band tune, perhaps thanks to its anthologization in Kerr's Merry Melodies around 1880 - this form is unusual for a Scottish tune in only having one strain. My guess is that the pipe tune derives from Tannahill's song rather than directly from the old Gaelic one.