And a bit more. Check the Farish website and those of other "N" manufacturers. Open wagons and some LMS covered vans are close enough to 1950s GNR(I) stock which ended up with CIE and was in use well into the seventies. Just sheep-dip in grey or brown (post-'72).
Mk 2AB coaches will be just a repaint of BR equivalents. I'm sure someone manufactures Mk 3's; same.
There's an LMS 0.6.0 tender engine which could be mildly altered to resemble an NCC equivalent, and I am sure I've seen LMS / BR Stanier tanks changed to approximately resemble UTA / NIR "WT" class.
Lining - especially by hand - will always look grotesquely overscale in "N" gauge, so a choice of livery where lining is either non-existent, or weathered to invisibility (think "Jeeps"!) might aid realism.
As others have said, a credible 800 can be made out of a "Royal Scot". I am sure with a bit of botching, a credible likeness to a J15 - almost essential for CIE steam era - or a "Woolwich", or even a 400 class if modelling the Cork main line, could be achieved. Plain unlined grey for all but the 800 would be an advantage!