That's exactly what it is. They were used on cattle specials and tended to be found lurking in Athlone, Mullingar and Broadstone when not in use. You would think that they also might have seen use on branchlines, especially where cattle was a big thing (e.g. Loughrea) but seemingly not so much. Certainly not in GSR times.
Senior took a photo of a similar van but without passenger accommodation at Broadstone in the early 1930s. It was shabby looking and as he said, probably (but not certainly) out of use. It was still in MGWR livery like the above. The full brake versions of these were green, so the above may be too. Failing that, certainly standard dark wagon grey.
There is a Cyril Fry model of a yoke of this nature in the Malahide Model Railway Museum. It is a very dark green, not unlike UTA coach green or BCDR loco green. The van that Senior saw in the 1930s was a dulled mid-green, but since anything in Midland livery in 1931 or 1932 hadn't seen a paintbrush for at least seven years, the original colour could have been very much darker indeed. Fry's livery interpretations are generally exceptionally accurate and exceptionally detailed, though there are a few aberrations to the theme; he painted a model of a 500 class 4.6.0, and two other locos, which spent their entire working lives in dark grey, in "Maedb"-style blue-green!