To answer a few points above, the RPSI is indeed registered as a charity and has been in the south for some 20 years, and in the north about 30. So, incidentally, is the DCDR, though that's another matter. The RPSI is also registered in the south (only) as a train operating company and thus, bona fide railway operator.
The C & L coaches at Dromod are sides and ends only and would require as big a rebuild as a new-build, if you know what I mean.
The derelict operation at Blennerville is subject to attention by a new group, who assure enquirers that they are making great strides, though nothing has happened, Local stories - which I have no way of verifying one way or another - seem to suggest that vested interests in the local authority may be making it difficult for what's really needed - a skilled enthusiast group - to take over. A carbon copy situation exists with the badly managed and equally defunct Foyle Valley operation in Derry.
18T and the C & L loco are indeed still in the USA, and the loco at any rate requires very substantial work, including a brand new boiler. The one member of their operation who took a great interest in it, having been involved in its move from here, has now passed away.
The remains of the SLNCR coach - one half if it - have now almost totally decomposed back to nature, along with the last remaining cattle truck, which fell apart about 15 years ago.
Based on the RPSI's and DCDR's experience, extremely few fare paying passengers are enthusiasts. Thus, exact accuracy isn't a big deal; if it was, who would travel behind a CIE diesel or CSET steam loco in a 70 class trailer on a part of the BCDR, or in a BR Mk.2 coach behind an NCC tank engine on the MGWR! Studies have shown that a Severn Valley style operation, with a seven-train-a-day timetable with maybe three main line steam locos out, would be utterly unrealistic for Ireland. Economically, for 5ft 3, as well as 3ft gauge steam, the optimum length of line here is 2-5 miles. Shorter, and you won't get as many people as you might, and you'll have to pitch the fare quite low. Beyond 5, and you'll find the public won't pay a fare sufficient to deal with track maintenance costs for a longer line, and more coal per journey.
You also need a good population centre.
Moyasta survives because Co. Clare is quite busy tourist wise. Tralee was a mess because it was run by a county council and was thus almost doomed to failure. Finntown has wobbled financially because, again, wages have to be paid; this is utterly unsustainable in a set-up like that. Downpatrick is sufficiently close to Belfast, just as Suir Valley is to Waterford plus a good tourist area. Suir Valley manages a longer run because of much lower operating costs; a turf or ESB diesel will run thrice round the world on the amount of diesel that would take A39 to Inch Abbey and back. An Alan Keef loco of that size will run for three years on a bottle of PC correct, eco-friendly, no-emission, low sugar, non-sectarian vegetable juice. May contain traces of nuts; do not try this at home.
And there we have it. Now; to old coach bodies - DCDR has numerous ones, but until and unless $$£$$£$£$£€€€€€€€€€€ appears, a laminate and 70 class trailer will operate public services. Accurate? No. Popular with the paying public? Yes.