The cost of the long-awaited MetroLink project could be substantially higher than the original €3bn figure.
Transport Infrastructure Ireland and the National Transport Authority said it is too early to estimate with certainty what the final cost will be.
A revised plan for the line has curtailed the southside section, meaning it will run from Swords, through Dublin Airport to a final stop on Charlemont Street, and not extend out to Sandyford.
70 homes, a pool and a gym are to be demolished to make way for the project.
MetroLink plans will now not affect Na Fianna and Ballymun Kickhams GAA clubs on Dublin's northside.
"There are very significant benefits associated with MetroLink, particularly in terms of the integrated transport system that it will bring about for Dublin," said Anne Graham, CEO of the NTA.
"For example, thanks to MetroLink, there will finally be a rail link to Dublin Airport, and with easy interchange with other modes including bus, Dart and commuter rail, MetroLink will make it easier than ever to move into and around the capital.
"It is also of benefit in providing a more sustainable transport alternative and by providing a great incentive to get people out of their cars and on to public transport.
"MetroLink will be a significant economic driver for the region with about 4,000 jobs to be created during construction.
"The MetroLink plan we are announcing today is a key component of the Transport Strategy for the Greater Dublin Area, 2016-2035, adopted by Government in 2016. In addition, MetroLink is one of three major public transport infrastructure projects included in the National Development Plan under Project Ireland 2040, published in 2018, along with Dart Expansion and BusConnects."