I think in many ways this is what IRM/Accurascale are doing with the likes of Mk2s and whatnot, wherever possible looking at commonalities to utilise.
What could be done is something akin to Genesis or Rapidos planked wagons, building something 'generically' Irish that with a different paint job would suit British stuff just as well. The problem with that of course is Ireland has a larger loading gauge.
For locomotives honestly might have a better chance looking at something like an 0-6-0 and which ones had the closest wheelbases to british ones. You of course have the Woolwiches too, though Rapido is currently doing the U, the larger dia driver variant.
Something that could be looked at, though perhaps not true to the OP proposal, is that with the acquiring of Heljan, a 'common' TT 120 chassis could be utilised for 00n3 Irish. We've seen Bachmann offering a wide range of chassis for kitbashing at the like, Rapido and Fourdees make strides in using a common chassis with a 3d printed body on their Sirdar, perhaps something similar using an 0-6-0 chassis which can have pony trucks and bogies added on front and back to cover a wide range of Irish narrow gauge motive power!