The Minister for Transport has been called on to take immediate steps to reduce cost overruns on the national roads programme, following a report which says the plan is nearly €10 billion over budget.
Mr Brennan was criticised following the publication of the Comptroller and Auditor General's investigation today.
Labour's spokeswoman on transport Ms Roisin Shortall called on Mr Brennan to move immediately to introduce measures to control what she described as the "spiralling cost" of the national roads programme.
"The C&AG's report is damning. Constant warnings were issued that the original €7 billion estimate was wholly inadequate for the extensive work that must be undertaken to upgrade Irish roads. Now the taxpayer is left to foot the bill for the National Roads Authority's incompetence," Ms Shortall said.
"Yet Minister Brennan must bear some responsibility. Despite oft-repeated promises, there is still no sign of the National Roads Infrastructure Bill, which if implemented correctly would cut the cost of land acquisition to assist road building.
"Similarly, the fees charged by consultant engineers working on the national roads programme can reach astronomical amounts, as they are guaranteed 4 per cent of the contract price, no matter if that price sky-rockets during the duration of the project they are working on."
The Transport Umbrella Group (TUG) said Mr Brennan should become "cost obsessed" in light of the C&AG report.
It said it had already predicted the total capital cost of the Dublin Port Tunnel would "easily top €1 billion".
"This clearly highlights the incompetence and misuse of funds in an area which is vital to our economy - the national road construction programme," TUG spokesman Mr Jerry Kirsey said.
"As we eagerly await news from the Minister of Transport on his decision to increase the height of the Port Tunnel, we learn of the total mismanagement by the NRA which is horrific. The board must consider their position in the light of the overall misuse of taxpayers money. It is time the minister became cost obsessed."
TUG has been lobbying for a change in the operational height of the Dublin Port Tunnel from 4.65metres to 4.90metres. The group consists of representatives from Irish Ferries, Stena Line, the Dublin Chamber of Commerce, the Irish Petroleum Industry Association, the Chartered Institute of Transport and Logistics, IBEC, Irish International Freight Association and the Irish Road Haulage Association.