I’ll try to put together a rough guide to what could be mostly expected to be seen in most places. With so many classes of loco having but one or two examples, the exceptions will far outweigh the norm, however few of us will have an example if every GSR loco in our possession.
Obviously, the J15 being by FAR the most common loco, was in many ways the 141 of the past. Few West Cork locos ever left their area at all, even as one-offs, though of course one of the famous “Bandon Tanks” ended up on the DSE suburban. The J15’s and their neighbours, the MGWR J18s, tended to stick to their respective areas, though J15s were very common on Tralee - Limerick - Sligo after the W & L came into the GSWR fold.
Most ex-Midland branchlines had J18s or G2 2.4.0s from opening to closure or dieselisation. Most ex-GSWR & WLWR branches ended up with J15s; many having them start to finish.
Michael McMahon’s GSR loco “bible” fills in the many, many blanks and exceptions, describing the operational habits of the lesser classes.
Certainly, just as any layout based on UTA simply can’t be realistic without a UG or a Jeep, anything NIR must have an 80-class or three, and CIE / IE from 1963 to 2000 simply won’t cut the mustard without at least one 141, nothing CIE 1945-63 looks right without a grubby J15.
The forthcoming “Dugort Harbour”, despite being just a small shunting terminus, will have three J15s and three 141s.....plus a few Cs, as befits many branch termini on CIE between 1955 and 1963.