Steve
As you know, Child ballads were not written by Child - they were not written by Child; they were a collection of all the ballads he could lay hands on at the time which he identified as 'ballads'.
It was not a finished work and was never considered as definitive - it is merely an invaluable guide to our understanding of the phenomenon.
The fact that we already have a number of ballads identified as 'ballads not in Child' ar an indication that we are still dealing with a 'work in progress'.
Four songs in Irish have been claimed as having enough similarities to be associated with four Child ballads - not having examined them closely, I am not prepared to accept a statement that 'there are no Celtic Child Ballads'
They do have similarities, which might make them versions of existing ballads - or it might make them 'supplementary versions' - or it might make them ballads on similar themes, as can be found say in the riddle ballads.
I honestly don't know, and I haven't the Irish to do the math.
It's a habit of mine to bristle at definitive statements when it comes to a 'work in progress' - whether it's ballads or origins of folk songs.
Sorry
Jim Carroll