Transport Minister Paschal Donohoe plans to carry out an in-depth analysis of how services are used. It will "spell out very clearly" the costs involved in keeping lines open and services running, he told the Irish Independent.
He said the move was part of developing a long-term strategy for land transport in Ireland. Given the enormous amounts of public money involved, passenger numbers across the network "have to increase", he added.
A report commissioned by the Department of Transport, 'A Strategic Framework for Investment in Land Transport', found annual investment in the rail network was €291m, and that the situation was "not financially sustainable" given low passenger numbers on some routes.
Mr Donohoe said the cost of operating the network had to be benchmarked against investment in other transport modes.
"The objective of that (€291m) investment was to ensure that the rail network we have at the moment can be kept open and accessible," he said.
"It would be my objective to commence a consultation process next year on railway policy in Ireland, and the use of it, and the opportunity cost.
"I want to spell out very clearly that we are committed to keeping open the rail lines we are using at the moment, but we do need to communicate to the broader public as to the quantity of investment required.
"I think it's very important that we have a broad discussion as to how our rail lines can be used.
"What we have we are going to keep, (but) demand and use on all these railway lines needs to increase."
Iarnrod Eireann has been hit by falling passenger numbers in recent years, with 37.617 million travelling last year, compared with 45.5 million at its peak in 2007.
This represents a modest increase of 2.4pc on 2013's figures, but concerns have been raised about three lines which carry few passengers.
They are the Limerick-Ballybrophy line, which carries just 23,000 people a year; Limerick Junction to Waterford, which carries 28,000; and the €160m Western Rail Corridor between Limerick and Galway, which opened in 2010 but carries just 50,000.
It is understood that the yearly cost of operating all three lines is €15m. This is despite the three lines carrying just 101,000 passengers over 12 months. By way of comparison, the Dart network in Dublin carries more than 44,000 people a day.
Transport sources said that among the issues was that while the Department of Transport acknowledged the network was expensive to operate and in need of investment, under-utilised lines were being kept open, which added to costs. In many cases, it is quicker to travel by road than rail. One source said the focus should be on "growing numbers on the lines you have".
Iarnrod Eireann operates 2,165km of railway line, and passengers travelled some 16.9 million kilometres last year.
Due to investment in the last decade the company now operates one of the most modern fleets in Europe, while a major resignalling project in Dublin will increase capacity through Connolly Station from 12 trains an hour to 20 from 2016.
The World Economic Forum competitiveness report ranked Ireland 31 out of 144 countries in terms of the quality of the railway infrastructure.
