I don't actually know. I used the term (as others do) to describe a rather vague period in the Olden Days! I believe the Druids were contemporaneous with both Celts and Romans - it needs a scholar to answer really.

I think the suggestion is that they had a strong language which somehow survives in unexpected places, such as nursery rhymes. If one of those is indeed song refrains, it might explain why some choruses are actually quite difficult, phoenetically, to sing - having unexpected 'words' which the requirements of mouth rhythm would not suggest.

Eg Tooraladdy whackfoladdy toora loora ling. Why 'Ling'? 'Ing' is a hard sound to sing well on a drawn note. Maybe singers were replicating a harp riff? But then why do you get 'whack' all the time? It's a different sound altogether to touralooraladdyooraloollydooa etc. And you get it with all the fiddlididdlyfoddledaddly sounds too.

I just found myself wondering if this was some remnant of language rather than sound for sound's sake.

I've nver been convinced about Non Sense (as Mervyn Peake called it) 'happening' in songs. A song-writer putting nonsense in a song is going to want it to be just as clever as his 'proper' words, surely?