According to the notes in the Cast's recording:
This song and tune were already old when, in the early 1700s, Allan Ramsay included it in The Gentle Shepherd and guaranteed its popularity. A few years after its appearance there, it became a fad among trained fiddlers to try and add second sections on to old traditional tunes. Thre's a story that the celebrated Italian composer Geminiani, then living in Dublin, had a go and gave it up in frustration after blotting several quires of paper.
The song Buain na Rainich is also known to be ancient. However, the chorus anmd the first verse are the only ones remembered along with the tunes. Broom o' the Cowden Knowes is one of three tunes which can be used with it. I have 6 verses, and it is confirmed that Kenneth MacLeod wrote them, probably at the end of the last century, or the beginning of the 20th. For the lyrics
Buain Na Rainich