"Worst fault in quite a number of years''.
Irish Rail says more than 20,000 passengers were delayed due to a major fault at Heuston Station at around 6am.
The company says the fault is one of the worst it has ever experienced.
Problems began when the signalling system, which ensures the safe movement of trains malfunctioned.
Services resumed after a temporary manual signalling system was put in place. Barry Kenny from Irish Rail said 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."
Knock on delays are still being experienced around the country.
Irish Rail says its unacceptable that passengers weren't informed directly that services had been suspended.
It says it will be reviewing what happened it terms of the fault and whether they could improve how they communicate any problems to passengers.
