I'm going to end up moving some vents and sanding off some ribs...
IRM have placed their 9 pairs of vents according this drawing, published in Autumn 1954. Unfortunately, Inchicore chose to fit 10 pairs of vents when they they built the real thing.
Some of IRM's roof ribs are in the right place.
To be fair, I haven't found any drawings showing the roof ribs, or the correct vent positions. But it can be worked out from careful study of photos - there is a logic to it.
Edit: I'll probably only bother re-doing the roof on one of my carriages, it's a lot of work for a very small change in appearance. But I want to do one with a green roof as originally delivered, so I'll make the effort to get that one right.