The Government's €34 billion, 10-year transport plan will be completed on time and on budget and will revolutionise the State's infrastructure, Taoiseach Bertie Ahern said.
However, the Opposition charged that much of the project has been promised before under the National Development Plan, which was to have been completed by the end of next year.
Emphasising the need for value for money, Minister for Finance Brian Cowen said each project would be tightly managed and would still require final approval from the Department of Finance before going ahead.
Under the Transport 21 plan, motorways will be completed between Dublin and the major cities by 2010, while a road of near-motorway standards would run between Cork and Letterkenny, Co Donegal, by 2013.
Billions are to be spent on trains, including "a New York-style Grand Central Station", to quote Mr Cowen, under St Stephen's Green, which will act as a hub for mainline, Luas and metro services.
A metro will run from near Tallaght to Swords on one line, and from Swords to Stephen's Green on the other, while extra Luas lines will run to Citywest and Lucan. The existing Sandyford line will run to Bray.
The plan was launched before an invited audience in Dublin Castle yesterday by Mr Ahern, Mr Cowen, Minister for Transport Martin Cullen and Tánaiste Mary Harney.
A tunnel between Heuston and Pearse stations in Dublin will link mainline services from the south with northern-bound services, and create space for extra commuter services.
Furthermore, diesel trains will not be able to use the tunnel, which, therefore, raises some issues about the number of commuter trains that can be put into service from provincial towns, an industry source said last night.
The huge scale of the work will mean traffic chaos in the capital's city centre for much of the next decade. "O'Connell Street will have to be dug up again," said one industry source last night.
The Department of the Environment will now face pressure from construction companies interested in the multibillion-euro contracts for tender closing dates and specifications.
Guaranteeing funds for the project, Mr Cowen said capital spending would run at 5 per cent annually for most of the next decade, though it would rise beyond that between 2008 and 2012. Currently, however, the Exchequer's ability to spend such a budget remains in question since it is failing to spend 5 per cent of its current income on infrastructure this year - falling short of projections by just under €1 billion.
Changes to An Bord Pleanála due under the Critical Infrastructure Bill will mean it will be able to handle the major planning applications created by the plan, said Mr Cowen.
The plan was agreed by the four key ministers several weeks ago and approved by the full Cabinet last week, following nearly a year of difficult negotiations between Finance and Transport.
Rejecting Opposition charges, the Taoiseach said Transport 21 had been "fully evaluated and built into budgetary planning over the next 10 years". Nearly €9.5 million would now be spent "every day" up to 2015 to give Ireland the services needed by a modern economy and expected by foreign investors in a country as prosperous as Ireland.
"We need and deserve a first-class transport system. We have all worked to generate the resources, now we must invest to continue to improve the lives of everybody," he said.
Ms Harney said it was "very ambitious and very radical" while Mr Cowen said it would involve a "massive and necessary commitment of resources for a key sector of our economy".
Mr Cullen, who has been working on the plan for the past year, said it would deliver an integrated transport system for Dublin as well as completing the inter-urban motorway network.
In Dublin, it promises seven new Luas projects, two metro lines and a city-centre underground rail link that could double the number of public transport users in the city.
Outside Dublin, the Government promises to reopen the Western Rail corridor on a phased basis from Ennis, Co Clare, at least as far as Claremorris, Co Mayo, as well as the provision of new rail commuter services in Cork and Galway.
Transport 21 also proposes upgrading roads connecting Letterkenny, Co Donegal, with Sligo, Galway, Limerick, Cork and Waterford, to create a route called the "Atlantic Corridor".