On the 13th of September, Steve said that the link Jeri posted, to a song sung by Cliff Haslam, was the tune he's writing about.

I liked the tune, so I've made a MIDI of it for myself and to post here. If Joe is available, the MIDI should appear here soon.

Cliff is singing in the belly-dance minor of the key of F, so the song has B flats and C sharps in it. It's in 6/8 time and starts with a pick-up note. Raising it to G would make it easier for folkies but would push the highest note up to E, not a popular note.
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I have a theory about 'derry down' choruses. I think they are supposed to be imitating the sound of a drum, so I try to sing them to sound like drum beats, not words. Steve, you can put that in your paper and get points for originality. As to your basic question, I have never heard the song before, so that includes in the 20th century.