Thanks very much to all you night owls - or should I say stars?
As usual in a long post, I missed stuff out. I did mean to add that the promising census record I found was the one in Bahana. (I also meant to ask about the best way to order BMD certs - over here GRO is now much cheaper than individual reg offices - and how to find baptisms if you don't know a parish).
My cousins are willing to spend but not to lash out on likely duds and this line looks promising but they are convinced they'll find certs in their Dad's papers - still waiting on that, I'd expect his marriage but not sure about his parents'.
This finding of Kate at the deaf and dumb school is very promising. I know Graiguenamanagh is in Kilkenny - oddly enough I passed through in 1987 not knowing the connection - but it's right on the Barrow and my memory is that some of it is over the border in Carlow. (Bahana near the border too). So when Kate joined the school they could have taken the county of the post town - this happens often in England and Wales.
I was amazed that you found birth records with actual birth dates for both girls. I only had the quarters. These are so nearly exact to the 1939 register and "Kathleen"'s death record that I feel they must be right. I couldn't find a death record for Elizabeth Davies even knowing her supposed exact DoB and place of death - I'll try again.
I'll try to persuade my cousins to send for any marriage cert not yet found and in any case post updates as soon as anything is known.
Thanks again, Chris