Many commuters travel from Sallins
Rail services operating into and out of Heuston Station in Dublin were suspended earlier , due to a major signal fault.
This included all InterCity services, the Portlaoise commuter service and the Newbridge/Hazelhatch to Grand Canal Dock service.
It will result in a knock-on effect for services this morning.
Efforts are being made to resolve the problem but it is not known how long it will take.
Thousands of commuters will be affected.
Commuters are advised to follow Iarnród Éireann on Twitter for updates.
The latest development is that services from Portlaoise and Hazelhatch to Grand Canal Dock remain suspended.
But all intercity services, including Galway/Westport are back operating.