The Minister for Transport, Mr Brennan, is to bring to cabinet in the next two months his final proposals for a metro link between Dublin city centre and the airport.
He is expected to tell colleagues that the link can be built for less than €2.5 billion, and could be open to its first passengers by the end of 2009.
However the proposals could be seriously hampered by the fact that the project is feasible only as the first stage in a much more ambitious 20-year underground scheme, costing up to €20 billion.
The Department of Finance is still opposed to the project on the basis of a report it commissioned which suggests that a much cheaper alternative exists, the extension of the Dart to the airport, via a new spur line.
The minister has now received indicative costs from the Rail Procurement Agency (RPA) that the Metro link could be built for €2.4 billion, half the figure he was given last year.
This could be done through halving the time for the planning stage of the project from four to two years with new legislation to speed up the planning process.
Legislation to transfer underground property rights to the State is also being prepared.
The route of the proposed line has yet to be finalised however. It will run overground from the Airport, through Ballymun, going underground at Dublin City University in Ballymun.
The final plan is being drafted to include a route passing through Connolly Station, Tara Street and St Stephen's Green at the insistence of the minister.
According to sources in the Department of Transport, Mr Brennan remains confident he can convince colleagues of the merits of the project, and that it remains a realistic proposal.
Last year the plan was put in jeopardy following an initial report from the RPA that the airport link would cost up to €4.8 billion, and take at least seven years to build.
This included four years of planning.
Despite the latest figures, the proposal is still facing considerable opposition from the Department of Finance, which commissioned its own report on a rail link to the airport.
The report has advocated a spur from the Dart line as an alternative.
Mr Brennan is believed to have told Government colleagues on the cabinet sub-committee on infrastructure that the project is a feasible when considered as the first stage of a larger metro scheme. It would greatly increase public transport capacity in the city, while the Dart spur would have a limited impact.
However, according to Government sources there is still a considerable barrier to the project over the overall spending commitment. The feasibility of financing through public-private partnerships is still under consideration.
"It would just be the first of ten planned phases over a 20-year period, so the Government would be committing itself to a much greater spend than €2.4 billion, and a lot of debt, whatever way it's financed,"said the sources.