IIRC the Irish market is quite a bit younger overall compared to the British market, 30-70 rather than 50-80. For some time here the 80s to early 2000s was the dominant era for modelling, as the rationalisation of the railway wasn't 'complete' until the delivery of the ICRs and the end of most freight in the late 2000s. Younger modellers like myself do tend in many cases to look towards older stock of the 50s and 60s which IRM is increasingly attending to. I would buy pretty much anything from that period if it passed through Mullingar and doesnt break the bank. When I can I'd like to buy a four-car ICR to give the odd modern twist on my layout.
A C class is a good go too, fills a big gap and have plenty rtr to pull behind them. The Jeeps do not have the same luxury in rtr, though leslie is quite happy to supply spoil wagons! Most Irish steam is in an awkward spot in terms of rtr rolling stock to pull, and/or no. of locos in a class make it difficult to run compared to an A or C class. The Jeeps and GNR 4-4-0s can get away with pulling CIE 60s stock though.