A Waterford to Mallow layout, yes please .
On the 121's, they ran for the best part of 50 years in everyday service. Id easily would take 6 or more and i think the majority of active modellers for the era's that they ran in would buy the same amount and in some cases even more. During their careers they've probably covered most if not all of the post 1961 railways on the Island, have done just about every job on the network from express Cork to Dublin services to pw trains in their later years. Livery wise they ve carried quite a few liveries grey/yellow, black/tan, supertrain, ir black/orange & IE black/orange.
Theres plenty of choice and if your in any doubt look at the amount of 071/111 models were released, nearly the same amount of real life locos existed 21 071/111's v 15 121's.
As pointed out already they ran in quite a few formations, from on their own, in doubles with other 121's, 141's, 181's. Plus push pull modes with mk3's, old railcar units especially near where there is an existing strong modelling base ie Dublin and they also done mk3 p/p out of Cork as well for a few years.
For my lot id be looking at 4 IE livery, 2( prob 3) in Ir livery and will probably go for a grey/yellow as well.