One of the bugbears that we all have to put up with is a lack of space which invariably means running unrealistically short trains, both passenger and freight.
Ofcourse, a good many of Ireland's passenger trains out in the sticks are made up of only 1/2 carriages plus BGSV, but mainline passenger trains tend to be just a bit longer. Now if you search for Bradfield Gloucester Square on youtube you will find an extremely realistic model with workings based on Bradford Forster Square and where the mainline passenger trains are but five carriages long. That seems to work.
Translated to Ireland, that would mean that a passenger train made up of Mk2's would use the five carriage types produced by Murphy Models.
Dunno about Mk3's since they tend to be so much longer anyway.
With regard to freight trains, one trick is to use 4w wagons rather than bogie ones where possible.
Stephen