There are delays of up to an hour to trains heading in and out of Heuston station after equipment was vandalised overnight.
Services through the Phoenix Park tunnel are also affected.
Irish Rail says a cable was stolen from its signalling system between Newbridge and Kildare, causing the delays.
Update: Our App and realtime departures on the website takes its information from the signalling system. As a result of this, information for services into & out of Heuston and Phoenix Park tunnel services is not accurate this morning
— Iarnród Éireann (@IrishRail) October 9, 2019
The transport company says services from Heuston to Portlaoise will terminate at Newbridge in an effort to minimise delays to trains into the city.
The problem is not expected to be fixed until later this morning.