If a new-build was to be a likelihood, like the RPSI's talk of another Jeep -v- a mogul, practicality would have to not just be more important than emotional preferences, but would have to oblitarate such thoughts.
In practical terms, the best options for a new build would be just about big enough to take the DCDR's maximum likely load (three to four bogies) over a distance of maybe 6 miles (9km), with least coal consumption.
We're looking at a Jinty perhaps, a GNR 2.4.2T, a MGWR J26 (ideally; simple reliable machines), or the West Cork "Argadeen" 2.6.0T. Pretty much all else else either would eat too much coal or would be too light (e.g. 90!). 90, of course, or the sugar locos, are perfect for current operation, the shortness of which makes even a Jinty of a J16 an unnecessary extravangance.
The RPSI's No. 3 "R H Smyth" was also an absolutely ideal locomotive for Downpatrick.