There simply isn't enough service on the Nenagh Line to make it viable. Not ever having experienced the charms of Roscrea castle, I may not be qualified to make sweeping judgements on it, but it might be difficult to pass the 8 hours and 3 minutes between the morning train and its return service.
Similarly, there is no realistic commuter service in either direction on the line nor is there a train that gets you to Dublin in time for a morning hospital appointment which are usually block booked at 9:30. So that rules out most of the other opportunities for generating traffic, leaving the line with the weekend student traffic and the odd shopping trip that of necessity lasts all day.
When Irish Rail upped the service frequency to 2 hourly on the Sligo line (and effectively hourly to Longford between 15:00 and 19:00), I thought they were mad as the services before then were hardly jammed. But it made a massive difference. They have a structured fare system where the day returns get progressive cheaper after the commuter trains in the morning and you can just turn up in Connolly to go home after you are done shopping. The result is that most of the afternoon trains are very busy and while the line may not be paying for itself in raw financial terms, it is generating valuable economic and social activity for the country. This kind of jump in frequency is the only way that the Nenagh Line is ever going to be a success and you can't blame the local population for not responding to a half-assed commuter service that seemed designed to prove that extra services don't work.
The common feature between both lines is that the only meaningful traffic originating in Dublin is the weekend student visits home. They have tried and failed with early-morning down services and matching evening returns from Sligo.