Irish Rail is currently working to fix the issue.
There are delays of up to 45 minutes for trains heading in and out of Heuston Station after equipment was vandalised overnight.
Irish Rail says a cable was stolen from its signalling system between Newbridge and Kildare.
Services through the Phoenix Park tunnel are also affected, and services from Heuston to Portlaoise will terminate at Newbridge.
Irish Rail spokesperson Jane Cregan says they're working to fix the issue: