The introduction of the new services follows a €13.7m investment
A rail tunnel in Dublin has re-opened to passenger trains for the first time in more than 100 years.
Commuters on the 6.14am train from Newbridge to Grand Canal Dock became the first to benefit from the new Phoenix Park Tunnel.
The introduction of the new services follows an investment of €13.7m in upgrading the tunnel route, funded by the Department of Transport through the National Transport Authority (NTA).
The line and tunnel were opened in 1877 and in recent years has been used for freight services, transfer of trains between maintenance depots, and special trains for events like matches, concerts and charters.
Regular peak commuter services have never operated through the tunnel in the modern era due to capacity constraints at Connolly Station, which have been addressed by recent resignalling works.
Irish Rail say seven new morning peak trains serving all stations from Newbridge/Hazelhatch to Grand Canal Dock - including Drumcondra, Connolly, Tara Street and Pearse - and eight new evening peak trains from Grand Canal Dock are running from Monday, with limited stop services in the opposite direction.
However they will not serve Heuston Station - customers for Heuston should use existing trains between Newbridge/Hazelhatch and Heuston Station.
Iarnrod Eireann say: "Services will also ensure connections at Hazelhatch/Newbridge for customers travelling on longer distance commuter services to/from stations such as Kildare, Portlaoise and others.
"The new services give a new direct public transport connection between stations in south-west Dublin and north Kildare and the central business district between Connolly and Grand Canal Dock.
"This is the first phase of services to operate via the tunnel. Iarnród Éireann and the National Transport Authority plan to expand services in the future to include off-peak and weekend services."
Welcoming the new service, Transport Minister Shane Ross said: "I'm delighted to announce the opening of this key piece of our public transport service.
"The Phoenix Park tunnel line is a vital part of our rail infrastructure and I know that it will prove to be of immense benefit to the travelling public, as will other transport projects such as the Luas Cross City line which will open in 2017."
The full line from Islandbridge to Glasnevin Junction is 2.75 miles long, and after the tunnel section continues through Cabra before joining the Maynooth line at Glasnevin Junction.
It has occasionally been used for scheduled Intercity services - most recently two Sunday services from Limerick and Galway which operated between June 1998 and September 2001.