Indeed, Mayner - I remember going through a 1960 WTT once with the intention of finding out exactly how many locos, crews etc were needed to do a day's work on that system. As you say, it was an odd working.
The Bandon tanks were indeed needed on beet specials. A recent DVD shows none other than an 800 class (802, I think) taking beet empties from Mallow to Cork, where a Bandon tank took over to ferry them to the junction. At that point a MGWR J26 (552) set sail with them for Clonakilty; the return journey being a mirror image as far as Cork anyway. Further interest could be added with 90 or 100 trailing 2 or 3 wagons into Courtmacsherry. If that's not fodder for a fascinating model railway setting, I don't know what is! Towards the end, beet was handled on the Cork - Clonakilty Junction with Bandon tanks or on occasion, a C class loco, but Midland J26 0.6.0Ts ruled the branch to the end.
You are right about the INW turns. There is no way they would have sent an "A" class out to Carrickmacross unless it was spare for a while in Dundalk. And it was generally (if not always) a GNR "A" class 0.6.0 which operated the "rump" INW goods. The locomotive used between Inny Junction and Cavan was generally a "C" class, so it may well have been a Mullingar pilot.
The railways were undergoing a fascinating period of change at the time, as we all know. I am surprised more model layouts don't concentrate on this period, but with the growing number of truly excellent kits available, there is no reason why this might not change in time.