Private owner wagons were exceptionally rare in Ireland. Unless I'm mistaken, the Kilkenny and Murphy ones are fictitious; I've never seen a photo of eitherf. The East Downshire Company had coal trucks on the Belfast and Co Down Railway, but there weren't many others. Instead, the railways would build wagons for private traffic, but they would almost always be in railway company livery.
The biggest thing on model Irish wagons to be aware of, probably, is that bought wagons all seem, without exception, to have black chassis as if they were passenger vehicles. Very few Irish wagons had black chassis (NCC "brown vans" were one exception, as they generally ran in passenger trains).
Prior to 1925, body (and matching chassis) colours varied from a very dark grey (almost black) as on the GSWR, to various shades of grey. Grey was universal among all companies 1925 - mid 1950s. At that stage, the GNR used brown on some stock, though most was grey; the UTA used brown on some in the 60s, and CIE started using brown in the 70s. Brown became the universal CIE colour after the elimination of loose-coupled goods stock in 1975 after cattle traffic ended.
In the era when a 121 would have run (60's - 2010's) four wheeled wagons would have been all grey, and no private owner wagons like Murphy or Kilkenny existed at all (so just use CIE plain grey for coal traffic!). As wooden four wheel opens were phased out in the late 60s, very few ever wore brown; they were taken over by Bullied steel wagons instead. These had unpainted steel bodies - a throwback to the unpainted silver "livery" of the 50s. Initially, chassis were the same, though in the 80s a few had brown chassis. Once doubled in height, at the end of the beet era, these were all brown.