The government could end up paying up to €200m to the Limerick Tunnel operators due to fewer cars using it than expected.
Around 24,500 vehicles pass through the tunnel each day.
The costs have branded "insane" by members of the Public Accounts Committee.
The Limerick Tunnel opened in 2010 to direct traffic away from the city, and so far has cost €31.4m.
The current contract to operate the tunnel runs until 2041 and under it, the state must make traffic guarantee payments to the operators.
These could reach €200m due to fewer vehicles than anticipated using the route - currently around 24,500 pass through the tunnel daily.
Labour TD Alan Kelly suggested this cost was "insane" at the Public Accounts Committee yesterday.
Michael Kennedy from Transport Infrastructure Ireland defended the payments saying the project was still viable as the traffic would otherwise end up in the city.
However, a cost-benefit analysis has yet to be published and PAC said it raises further concerns about the value for money of public-private partnerships.
- Digital Desk