The Red Line runs in an east-west direction through the city centre, north of the River Liffey, before and travelling southwest to Tallaght, with a fork to Citywest and Saggart.
The Red Line opened on 26 September 2004.
Construction work began in March 2001 on the Tallaght to Connolly line, as well as the Sandyford to St. Stephen's Green section of the second line, with Ansaldo of Italy and MVM of Australia getting the contract to build the system. The St. Stephen's Green to Dublin Airport section was dropped before construction began, as it was decided to serve the area by a metro instead. The contract to maintain operate the system was awarded to Veolia Transport Ireland (formerly known as Connex).