David-re the bread containers,check out Provincial wagons website under GNR(I) wagons,Leslie has produced all of the bread containers that ran on the GNRI for 00 scale,I know not your scale but the pictures show their colours.Although the bread containers all relate to Ulster bakeries and most of this traffic would have been contained within Northern Ireland,Leslie states some did stray into Cavan,Monaghan and Donegal.This would make sense as the GNRI lines crossed the border,with some lines going back and forth along the border between each country,so I think you would be ok having bread vans.There is a link to Provincial Wagons website at the bottom of the home page in the manufacturers section.
With regard the butter vans,I am positive they ran on both sides of the border,so I think you would be ok modelling one of these also.