Can't see any RTR steam locos being turned out any time in the near future, I'm afraid.
As Horsetan pointed out, a J15 just might shade it because there were so many of them and because they had a long innings, plus two have survived and have run in the diesel era on specials. The WT would be a riskier investment as, though they lasted in service into the early '70s, and No 4 is still knocking around, they were a small class and, judging from modellers I know and what I've seen at exhibitions, the Northern Irish steam era is a minority within a minority within a minority among modellers here.
An AEC railcar set could possibly have some mileage, as they carried a wide range of liveries (Two variants of CIÉ green, black and tan, GNR and UTA), turned up in all corners of the island, and ran from the 1950s through to the early '80s either as powered or hauled (push-pull) stock.