An actual GWR design was unlike anything Irish, so wouldn't do. The above looks like a brown and cream Mk 1, rather than an actual GWR coach.
Mk 1's are really only suitable for genny vans. The outline is not only unlike anything Irish, but because British Railways Mk 1s are so commonly "out there" and obviously associated with Britain, they are an onbious "botch job" and don't fit in - in my humble opinion - with Ireland.
If you WERE stuck and just wanted to paint one in CIE livery, I would retain the loo window vents but fill in the windows - at least.
Now, the LMS style would much better suit both final NCC days / early UTA coaches, or GSR "Bredins" or 1950/1 CIE stock which was more or less "later Bredins".
The circular window would have to go.
Brake passenger coaches were more common in Britain than here, and there were no Bredin stock with half passenger accommodation and half van, and few steel-sided examples of the species until late 1960s conversions of earlier stock (for example the 32XX series CIE brake standards). Thus, I would use all-passenger stock for conversion to "roughly-CIE-looking" stuff.
In place of the circular window, one might add a toplight vent like in the loos on laminate stock. Not an accurate representation of anything, of course, but a simple conversion that would fit in as well as anything not strictly correct.