I saw a decomposing body of one of those about forty years ago in a field near Barna, outside Galway. It was sheeting on top of the older planks. All was rotting, so it was impossible to tell what condition the planking would have been when they initially put the sheeting on. From my own observations over the years, i believe that some sheeted conversions were done in a heavy-duty plywood, and others were something like sheet steel.
As for the outside-planked ones, a few comments. Apparently, this was the terminology used to describe them on the railway - certainly, in Inchicore. I am not sure they were ever intended to be insulated vans per se - I feel that Senior would have told me that if it was the case. I remember them in traffic, and they were just used as normal goods vans, to be mixed in with the other standards of the day - GNR wooden vans, GNR cement vans, CIE Palvans, CIE "H"s and these. I can't help feeling that iof there ever was an intention to actually use them as "insulated" vans, some exterior markings to that effect might have been evident.
Finally, they were conceived in GSR times, but I cannot recall ever seeing a picture of one with "G S" on it. The wagon plates on them, though, had "G S R" on them.
(And talking of wagon plates, I saw a GNR cement van in the 70s, now in CIE brown, and obviously used as a standard "H", and it had a standard CIE wagon plate with "C I E", and the number 111N. I wish I had been able to get one of those plates....!)