The comparison with "Wop Bop a Loo Mop a Lop Bam Boom!" and "Da Doo Ron Ron" is interesting, but not very helpful.
Both of those have a traceable origin.
Little Richard was singing the drum fill that he wanted to start the song with. Any drummer could play that.
"Da doo ron ron" is "they do run run" from an old children's song, a version of Three blind mice.
Any song writer would know not to start the chorus of a song with a meaningless fill. Actually, I have always wondered how many of the "To me rye fol..." and "Tourelay..." choruses in folk are actually the record of the collectors writing down some old buffer's attempt to sing the instrumental break. Most villages would have had a band for Church and Pub, and I just can't imagine a pub full of drunken Englishmen singing nonsense words (Except folkies of course, who are daft enough for anything!)
Why does the "Jolly (Johnny?)Rumbelow" get so little attention? The phrase does turn up in shanties, often as a name. I don't know of any Celtic deities or pre-christian spirits with a name that sounds similar. I think the first line needs to be looked at as a whole.
If it's not Cornish of Breton, does anyone know Basque? (not a sort of corset, calm down) supposed to be the basis of the secret language of witches.