Local TDs Pat Carey of Fianna Fáil and Roisin Shorthall of Labour said the decision to go underground, rather than though the centre of Ballymun, was "best for Metro, best for transport and best for the Ballymun community," and was a "victory for local residents and community groups".
Ballymun residents had been opposed to the Metro line running through the centre of the town - on ground level, in a trough or on an elevated line - on the grounds of traffic congestion and safety.
The 17-kilometre Metro North line, part of the Government's Transport 21 strategy, will run from St Stephen's Green to Swords, stopping at O'Connell Bridge, The Mater Hospital, Croke Park, Dublin City University and Dublin Airport lie en route.
It is expected that the journey time on the line from St Stephen's Green to the airport will be 17 minutes, with a 24-minute journey time to Swords. Trains are expected to run every four minutes at peak time, carrying up to 30 million passengers a year.
Construction of the Metro North line is due to start in February 2009, and completion is planned within four years; 2,000 "park-and-ride" spaces are also planned.
The RPA is preparing an Environmental Impact Statement on the project.