The Public Accounts Committee has been told that over €3bn has been collected in carbon tax since 2010.
The Chair of the PAC, Fianna Fáil's Sean Fleming, said the State raised over €400m through carbon tax last year, but he said it is not clear how it has been spent.
He said: "We need to know from the Department of Communications and Climate Change and the Department of Public Expenditure and Reform where that money was spent, because it would be shameful to think it's been collected on carbon tax and has not been spent on reducing CO2 emissions issues."
Deputy Fleming cited a report from the CSO which he said showed that the State had incurred significant costs in terms of environmental damage and foregone taxes through subsidising turf and coal.
He said that the revenue raised from the tax should be used to come up with an alternative, in order to reduce people's dependence on coal and turf.
He said people want to know where the money from the carbon tax is going, and he said it was the PAC's job to do this.
Sinn Féin's David Cullinane told the Committee the tax was meant to help change people's behavior, but he said there was no evidence that this was the case.
Separately, representatives from the National Transport Authority appeared before the Committee today.
The Authority was criticised by the Committee for employing more contract staff than permanent workers.
The Committee heard that over 50% of the day-to-day workforce in the NTA are contract staff, which costs an extra €10m a year.
Philip L'Estrange, Director of Finance and Corporate Service with the NTA, said there were 140 permanent employees and an additional 149 people were on contracts.
Deputy Cullinane said there were very few public organisations where the numbers of contract workers were that high, and he said it was "extraordinary".
Deputy Fleming said the Committee had never seen any State body where over 50% of the day-to-day workforce were on contract.
He said the cost of going this route was "detrimental" to all involved.
Deputy Cullinane asked the NTA if there had been a cost saving analysis done to see what savings could be made if more workers were on directly employed contracts.
CEO of the NTA, Anne Graham, said there could be a saving of €10m every year if this was the case.
An official in the Department of Transport, Tourism and Sport, Deirdre Hanlon, told the committee that the NTA had carried out a review and agreed to increase their permanent staffing over time.
Ms Graham also said there is an urgent need for additional rail fleet to meet the increase in passenger numbers.
She told the Committee that the commuter rail system will experience capacity issues at peak travel times in the short term due to the improvement in the economy.
Ms Graham said preparations for the procurement of new electric and battery electric rail fleet are underway, but she said building rail fleet would take a long time.
She said the NTA and Iarnród Éireann had prepared a business case for the purchase of an additional 41 intercity rail cars.
She also told the Committee that the Authority has been significantly under-resourced, with the number of staff well below what is required.