By all means! There were enough ‘might have beens’ in the minds of Victorian railway planners to add another. Irish lines frequently exchanged rolling stock, so you could easily ‘cascade’ recognisable locos and wagons etc. There were even a few cases of standard U.K. designs being used (such as the LNW tanks on the DWWR), and the forthcoming CBSC 0-6-0Ts from OO Works. If the track gauge is correct, which you have sorted, and the scenery is right, that’s half the ‘Irish’ side of the picture anyway.