The problem isn't the number of users, but the investment required to get to a reasonable user base
Based on figures for other means of getting between the two cities and how they relate to travel to Dublin, you'd reckon that if the same standards of service could be provided as on the Dublin-Cork line (direct trains, average speed aboce 100 kph, hourly frequency), you could achieve decent passenger figures. My own estimates are
Galway-Limerick: 380K pa
Galway-Cork: 240K pa
Galway-Ennis: 170K pa
Galway-Waterford: 45K pa
But clearly on those figures you couldn't justify hourly 100kph trains from Galway to Cork, even before you consider the capital costs
So let's say we connect everything at Limerick Junction. Studies in Holland have suggested that 40% of passengers won't use a service once there is a connection involved (and that's with Dutch 15 minute frequencies)
So that alone loses 110K potential passengers to Cork and Waterford.
Now we're at 725K.
So we only need 1/3 of the frequency. But with that we'll lose about half the numbers. 360K.
So, even allowing that Limerick-Cork is in decent enough shape, could the investment to bring the line up to decent speeds be justified for 360K passengers. Lines to Galway, Wexford, Kerry, Waterford etc. could do a lot more with the same money.
I'm not an expert on train capital costs, but comparing to per km costs on other projects, I think this would be around 400m-500m. That's 1,500 per potential passenger per year and 15,000 per current passenger per year. Those are absurdly high figures considering the fare on the line.
The only real argument that can be made in the short-term is to identify quick wins, get them implemented and start getting minutes shaved off. Plus, sort out the situation at Limerick Junction.
People on the line should be happy that they got a station in Oranmore. Blarney has been waiting far longer with potentially higher passenger numbers.