The European Investment Bank has agreed in principle to contribute €500m to the Dublin Metro project.
The planned Metro would provide a 19km link from Dublin city centre, via the airport, to Swords in the north of the county.
The EIB said today that it sees the Metro as a key infrastructure project for the country. Its total cost is estimated at around €6 billion.
The funds are subject to full board approval, and a decision of the Government here to go ahead with the scheme.
The bank is also currently considering two other public-private partnership projects in Ireland, which would form part of a second western transport corridor between Cork, Limerick and Galway - the N17-N18 Gort to Tuam motorway link and the N11-N7 motorway.
Meanwhile, the European Investment Bank today reiterated its commitment to Ireland and said it would continue to support projects in the transport, energy and education sectors.
It also said it would reinforce support for small and medium sized businesses in close co-operation with local banks.
Funding of €1.02 billion for Ireland last year
The EIB last year provided €1.02 billion for six projects here - the largest ever amount secured by the country.
Funding for recent energy projects included €300m for the Eirgrid East West Interconnector and €200m for wind farms under the ESB's renewables programme. €300m was also given to Dublin airport for its new terminal.
The EIB also gave a total of €260m to AIB, Bank of Ireland and Ulster Bank for credit for small and medium sized enterprises during the year.
'We are confident that industry, transport, social infrastructure, health and education will continue to benefit from EIB support in coming years,'the bank's vice president Plutarchos Sakellaris said at the start of a visit to Ireland.
Mr Sakellaris met Finance Minister Brian Lenihan, the Governor the Central Bank, Patrick Honohan, Transport Minister Noel Dempsey as well as officials from the Financial Regulatory Authority during his visit to Dublin.