The multi-billion-euro Metro North project in Dublin has been given the green light by An Bord Pleanála.
The rail link from St Stephen's Green to Swords now faces final approval from the Government on a cost benefit analysis.
The infrastructure project is described as the biggest in the history of the State and the ruling from An Bord Pleanála runs to 1,700 pages.
The board has given permission for an underground track from St Stephen's Green to north of Ballymun where it will cross the M50 on a flyover.
It will go underground at Dublin Airport stopping at a centralised transport hub before going overground again to Swords with some of the line on stilts due to the undulating landscape.
The board has eliminated three stops in the Swords area at Belinstown, Lissenhall and Seatown, and ordered the relocation of a depot and park and ride facility.
The board wants the park and ride facility and depot moved from Belinstown, which is north of Swords, because of the risk of flooding.
The overall 18km line has therefore been shortened by 2.3km.
It wants the park and ride moved to near Malahide Estuary and the depot to Dardistown near Dublin Airport.
The final plan for the underground section at Ballymun and the stop at O'Connell Street also need to be finalised.
But it is the proposed 'big dig' in the city centre, involving moving statues like the O'Connell monument and closing off part of St Stephen's Green, that is causing concern to some businesses.
Enabling works on underground utility lines is due to start next spring, while the construction itself is scheduled to last from 2012 to 2016.
The overall estimated cost has varied from €5bn at the height of the boom to €3bn now with reduced construction costs.
Supporters point out that because it will be a public-private partnership the initial cost will be taken by the private operator. The Rail Procurement Agency will have to make a final decision between two consortiums - Celtic Metro Group and Metro Express - in coming months.
A number of economic studies have been carried out by the RPA with the latest showing that for every €1 spent on the Metro there will be €2 back in terms of overall economic benefit.
Meanwhile, Tánaiste Mary Coughlan has said the funding for Metro North 'is still a matter for consideration by the Government'.
Asked if funding for the project would now be ringfenced to ensure it went ahead, following today's decision, Ms Coughlan said this was only a Bord Pleanála decision.
She said the issue for whether there will be the financial wherewithal to provide Metro North was still a matter for consideration by the Government.
Labour leader Eamon Gilmore has said the Metro North project should be postponed, while Transport Minister Noel Dempsey has said it will go ahead subject to a final cost analysis.
The RPA says the line will be able to carry 20,000 passengers an hour with 10km underground providing a journey time of 20 minutes from Dublin Airport to the city centre.
The National Transport Authority has welcomed An Bord Pleanála's decision.
It said high quality integrated public transport was a key feature of successful cities worldwide.
The success over the last decade of quality bus corridors, the large increase in commuter rail patronage and the large numbers on the Luas Green and Red lines show the strong demand for fast, new public transport solutions, it said in a statement.