No, they were fully GNR owned, though yes, "de facto" used only for Guinness traffic as far as is known.
When modelling the past, we see numerous British Hornby or Bachmann models of private owner wagons. If modelling Irish prototypes, it must be remembered if accuracy is wished for, that private owner wagons in Ireland were very rare indeed, and many extremely short lived; also, they tended to operate over one specific route, not in general goods trains all over the place. Example: Downshire wagons - Dundrum (Co Down) to Belfast only. You wouldn't get one of these, for example, in Enniskillen, Mullingar or West Cork, and there would be no Guinness grain vans on the NCC!