Irish Rail has advised that all trains scheduled to depart between 06:00 and 09:00 BST local time on Friday have been cancelled.
It said that the service would resume at 09:00 local time but there would be further disruption.
More information is available at the Irish Rail website.
The 07:35 Dublin to Belfast train was not operating between Dublin and Newry. The 10:35 Belfast to Dublin train is not expected to run between Newry and Dublin.
The National Bus and Rail Union (NBRU) and the Services Industrial Professional and Technical Union (SIPTU) were in lengthy talks with the firm on Wednesday and Thursday.
Both unions called on Irish Rail management to address the issue of past productivity delivered by union members.
SIPTU organiser, Paul Cullen, said industrial action was "inevitable" if a "meaningful solution" was not found.
A three-hour rail strike is taking place in the Republic of Ireland after negotiations broke down on Thursday night.