Mayner & Minister; correct on all counts.
I have a picture somewhere of the stainless steel "demonstrator", including interior; must try to find it (now there's a challenge).
Before discovering this picture on, actually, a CIE publicity leaflet, I'd never even heard of it. It can't have been in Ireland long.
The tin vans and SOME (by no news all) laminates were indeed the only stuff to into traffic unpainted, unless you count the bodies, and done chassis, of the corrugated Bullied open wagons.
Plus, of course, the A, C and G601 class locomotives.
Park Royals were all introduced in green, with silver bogies. Without checking, I can't remember if the roofs were originally silver, but the silver bogies soon were repainted black.
On account of the ribs on the sides, Park Royals never carried the "flying snail".