Rail services on a number of routes were delayed this morning after an overnight signalling telecommunications fault.
Irish Rail said services on the northern commuter and Malahide Dart lines were suspended for a time but they have now been restored.
The Sligo line service has also resumed but is operating with delays of 20-40 minutes, the company said. It is expected the line will operate normally from lunchtime onwards.
Services on the Clonsilla-Docklands line were cancelled earlier and the trains were transferred to serve the Maynooth commuter route.
Dublin Bus accepted rail tickets in the affected north Dublin areas and bus transfers were in operation.
The company said it was working to restore services as quickly as possible and it apologised for any inconvenience to passengers.