The 7.5km stretch of railway links Dunboyne, Co Meath, with Dublin city centre.
The line was first opened 148 years ago, but closed 63 years ago.
The new line connects Dublin city centre with two stations around Dunboyne - one in the town and the second just outside, which has a park-and-ride facility for 1,200 cars.
Minister for Transport and Meath TD Noel Dempsey officially opened the line this morning. The service will be open to the public from tomorrow.
However, there is uncertainty surrounding phase two of the project, which would extend the line out to Navan.
The €500m project was not included in the Government's list of priority capital projects, leading Fine Gael to say the project has effectively been shelved.
However, Minister Dempsey has said the money is there and it will be completed close to the 2015 deadline.
