The new Luas Cross City service has been launched by Taoiseach Leo Varadkar.
The extension of the green line from St Stephen's Green to Broombridge in Cabra is expected to add up to 10 million passenger journeys to the service.
Mr Varadkar said the new line will regenerate areas of Dublin's north city and suburbs, and link up with the Maynooth line allowing commuters from Kildare to access the Luas.
Mr Varadkar said that given the economy is strong again, the metro north and the M20 motorway between Cork and Limerick are next on the agenda.
Minster for Transport Shane Ross and Minister for Finance Pascal Donohoe were also at the launch.
The new line will add 5.6 kilometres of track and 13 new stops also linking the new DIT campus at Grangegorman.
🚊 The Luas Cross City service begins at 2pm tomorrow 🚊 pic.twitter.com/OUV9rbjYYQ— RTÉ News (@rtenews) December 8, 2017
Safety warnings have been issued ahead of the commencement of the service.
Service operator Transdev has warned that trams will be going through the busiest part of the city centre up to every three minutes.
Part of the line loops back around the core city centre area and will operate every three to six minutes at peak times.
This will allow a connection to the Luas red line, while another connects the city centre with the Maynooth Commute line at Broombridge station in Cabra.
This will operate initially every 10-15 minutes at peak times.
Gardaí have urged road users to follow road signage and to be aware of the new line.
Passengers can avail of a special €1 fare for off-peak city centre journeys in the first 12 months.
New longer 55 metre trams are due to be in service by next April.