It will look great in black'n'tan and would not be a million miles off the solitary Bredin all-first (was it No. 1900?). "Ordinary" Bredins tended to have loos / doors in the middle, so this is your 1st class!
That said, with several varieties of laminates, three versions of Bredins with two different side profiles, and several varieties - many further rebuilt - many further HEAVILY rebuilt - of the 1950/1/2 CIE Bredin design coaches, AND the fact that in that era barely two carriages were the same in most trains, the sky's the limit. Thus, a technically non-suthentic coach in CIE green or black'n'tan tends to fit very convincingly among a mish-mash of other types.
I was going through pictures over the weekend for the next book, which show the grey / green / silver era (let's coin a phrase; GGS era!), and not only is there this variety, it also extends (in the 50s) to the inclusion of all shapes, sizes and types of wooden carriages, from 1920s main line GSWR to late 1870s gas lit six wheelers of (mostly) GSWR or MGWR background.
One photo I looked at had a green Park Royal, a GSWR 6-wheeler (green, of course), a silver laminate (filthy despite newness), a filthy silver tin van and a green wooden (GSW) brake. Another had a 1905 GSWR branch line bogies third flanked by (on each end) a brand spanking new tin van!
For reasons too many to expound on here, few DSER types lasted long into CIE days. They were non-standard in many ways and decisions at Inchicore were very much GSW-biased! Also, the DSER was already short of stock, and many weren't in the best shape. Even the GSWR had flooded the Harcourt Street line with GSWR and MGWR stock.....