cg - a few answers, hopefully. The follow up to "Rails Through the West" will be another odyssey through 1960s to 1990s Black and Tan country, with a well known but comparatively obscure cross country route featuring. Expect some green as well as b'n't.
With regard to your Ratio coaches, I think I know the ones you mean, ad they do indeed pass a reasonable resemblance to some GSWR types. The GSWR had only a few clerestory roofed coaches, and fewer still would still have been in traffic in the fifties, do you might consider flat roof replacements if the ones you buy are clerestory. If it is the bow-ended model, this means WLWR origin, as this is the only Irish railway company to have curved lower ends on carriages (as opposed to sides, of course!).
As for livery. If it is set pre 1955, you could have an occasional GSR liveried coach. The few pics I have seen of such things show the maroon faded to a pinky reddish brown (covered in soot and brake dust, no doubt) by this stage. Most, though would be painted the new (1945) CIE green, with full light green lining above and below windows, this plus snails and numerals edged in gold. The light green cane later, so if you are post 1955 or so, only older wooden carriages are still this dark shade.
On the West Cork system, a few old CBSCR coaches acquired plain dark green with no lining, and two snails instead of one. At least one appeared to have its snails in white instead of light green.
Post 1955, any wooden carriages being reprinted acquired the livery currently on the RPSI's Dublin heritage set, or DCDRs TPO. That us, a lighter green with a thinner and even lighter green unlined waistline, and unlined numerals and snail. A few were turned out without snails.
Chassis were black. Ends were black - never green of either shade. Roofs were dark grey.
Pullmans remained dark green to the end. It would have been interesting to see what one would have looked like in black n tan!