Update 12.20pm: This morning's train disruption is one of the worst Irish Rail has ever experienced.
Over 20,000 passengers were delayed when services were suspended at Heuston Station.
The signalling system, which ensures the safe movement of trains, suffered a major fault shortly before 6am.
Services resumed after a temporary manual signalling system was put in place.
Barry Kenny from Irish Rail says they've yet to fix the main fault.
"It certainly was the worst fault that we've experienced in quite a number of years in terms of severity, the time time that it happened and the impact on services.
"We're still working to rectify the main fault which is in the computerised signalling system which controls all of these routes.
"We'll continue to do that, rectify that, understand what happened to ensure we can prevent it happening again."
Update 8.21am: Everything departing Heuston station now will be on time.
Four to five intercity services from Cork, Limerick, Waterford that departed before the fault happened will be "significantly delayed".
The major signal fault has still not been rectified, a manual signalling system is now being operated.
It took some time to get up and running as "signalling is extremely safety critical", Irish Rail spokesperson Jane Cregan told RTE radio's Morning Ireland.
"There will be knock-on delays into and out of Heuston until mid-morning."
Update 8.10am: Limited train services are now moving despite the signal fault in Heuston Station.
The services operating through a manual signalling system are:
Nothing else is moving at present.
Update 7.19am: Irish Rail has admitted that there are “thousands and thousands” of commuters affected by this morning’s signal failure at Heuston Station, writes Vivienne Clarke
There are currently no services operating into and out of Heuston.
Spokesperson Jane Cregan told RTE radio’s Morning Ireland that the volume of passengers impacted would make it impossible to provide a substitute bus service as is normal practice.
Ms Cregan acknowledged that some passengers are effectively stranded on trains that had already left their stations of origin, but the majority had not yet left the stations.
“We are advising them to seek alternative transport. Because the signal system is down we can’t do anything.”
Due to a major signal fault. There are currently no services operating into and out of Heuston. This includes Newbridge/Hazelhatch to Grand Canal Dock services. DART, Commuter and Intercity services into and out of Connolly are not impacted. Update to follow— Iarnród Éireann (@IrishRail) May 28, 2019
The “major signal fault” means that no trains are coming into or out of Dublin’s Heuston Station.
“At present we’re working to rectify the fault.”
She explained that a number of services had commenced before the fault was detected so the early morning train from Cork to Dublin is stranded in Charleville while the Waterford train is in Kilkenny.
“In reality there’s nothing moving. We’re not in a position to provide bus services because of the numbers.”