There are delays of nearly an hour-and-a-half on all train services out of Heuston Station in Dublin.
Delays of up to 30 minutes are expected on to all services into Heuston.
Update: Expected delays of up to 80 mins to all services out of Heuston at present, and delays of up to 30 mins to all services into Heuston.— Iarnród Éireann (@IrishRail) February 25, 2019
Irish Rail said the issue is caused by "mechanical issues" between Sallins and Newbridge.
It is also causing delays of 30 minutes on the Newbridge to Grand Canal Dock route.
Update: Disruption to all services in and out of Heuston at present due to mechanical issues between Sallins and Newbridge. Passengers for GCD to join alternative services to Hazelhatch and change for GCD there.— Iarnród Éireann (@IrishRail) February 25, 2019
"There was a mechanical fault on an out-of-service train that was heading out of Heuston to take up a commuter service," said Irish Rail spokesperson Barry Kenny.
"That was on the outbound, that's why trains that are leaving Heuston are most severely delayed, delays between 60 and 80 minutes.
"In the opposite direction, which of course is where the maximum number of commuters are travelling, it is typically about a 30-minute delay, so a very significant affect to those customers."