People like that always want there to be an "ancient Gaelic original," and it's no use telling them when they're talking rubbish. That kind of ignorance is invincible. I wrote a crushing little reply at the time, but didn't post it.

Chi mi na Mòr-bheanna was written (as Dùil ri Baile Chaolais fhaicinn) by John Cameron of Ballachulish (c.1856). The tune wasn't originally Gaelic; nor is it "ancient." It's an adaptation of Johnny's so long at the Fair, which itself probably isn't older than the second half of the 18th century, and may not even be Scottish.