Planxty's own testimony on The Drummer Boy is that is came from Franke Harte, an Irish collector of songs.

"Frank is referred to by members of Planxty in the biography of the band by Leagues O'Toole, 'The Humours of Planxty' as a source of songs.

""The Little Drummer" was a song passed on by the late, great Dublin singer and collector, Frank Harte. 'He is perhaps the single most important collector of songs,' says Christy.
'I remember Christy and myself going up to Frank Harte for songs,' adds Andy. 'I'd known Frank since very early in my career. He was an architect living in Chapelizod and I first met him in about 1963. He was always slightly to one side. It would be Johnny Moynihan and myself and our clique, and Ronnie Drew and The Dubliners, all more or less of the same age, and Frank was probably seven or eight years older than I was. I liked him a lot.'[5]""

Se http://en.wikipedia.org/wiki/Frank_Harte which in turn cites ^ O'Toole, Leagues, The Humours of Planxty, Hachette, 2006, p197 ISBN 9780340837979.

A