As long as the line is manually signalled there is a high labour cost and as long as line speeds remain low the bus is going to murder rail because it can deliver more direct service to UL/Plassey Tech Park and Limerick city centre. Rail has virtually no outstanding use case in the Dublin-Roscrea-Limerick corridor - that's not me being pejorative, it's a fact.
NTA Journey Planner Limerick Train Station-Roscrea
BUS12 0529 0657 88min
RAIL 0630 0832 122min*
BUS12 0729 0842 73min
BUS12 0829 0957 88min
* 12mins of the train time involves walking from the station into town.
Until public transport in Ireland is recast into a complementary rather than cutthroat environment I don't see how a service like Nenagh finds a niche.