"Kilkelly" spins the tune out because it more syllables to fit in, but it's a straightforward variation.

"Mairi bhan og" is and was a very popular song, and most songwriters in the British Isles would have heard it, in the standard Scottish version. I was wondering if there was an Irish variant that would have been even more familiar to Peter Jones - it's an old enough song that I doubt its origins can be pinned down to one side of the Irish Sea or the other. (contemplator.com ascribes the tune to Duncan Ban Macintyre, which I don't think is likely).