On the main lines nowadays the only steam locomotives which are feasible to opetare are those which can do all of the following:
1. Haul a train with sufficient seats to pay the colossal cost of the operation, between IE charges, crews and coal. That instantly rules out several RPSI mainline locois, never mind shunters.
2. Haul a train at a sufficient line speed to fit into a timetable slot. This also rules out many RPSI locos which are bigger and faster than shunters.
3. Have sufficient coal and water capacity to travel longer distances than any shunter 9or 90) were built for.
Gone are the days when many rural lines here had only one or two trains - or none - on a Sunday, and perhaps only 3 or 4 each way on a weekday. The railways are (happily) getting busier by the day, and with the population predicted toi go on rising for the next 20-30 years at least, this one ain't gonna get easier, Consequently, the emphasis will increasingly be on reliable larger locos which can keep time.
Downpatrick os the place for 5'3" gauge steam long term. There really is no other likelihood of that anywhere. Plus, technically it's "main line"; the line from Downpatrick out as far as where the Inch Abbey line diverges to the right, plus North Junction to the southern limit of the line, is part of the former BCDR main line from Belfast to Newcastle! No. 90 will return to traffic as soon as finance and manpower allows there, and there are always the two Sugar Co. locos. The RPSI's two no. 3s have both worked there in the past - in fact the Guinness loco was the first steam loco to operate there in preserved days.
Bottom line: 3BG, LPHC3, CSET3, 1 & 90 will not be able to operate tours on the NIR / IE systems again.