Plans to run Dart services every 10 minutes have been derailed.
Rail chiefs said they are shelving the proposed increased frequency over a row with union bosses.
Transport Minister Paschal Donohoe last year announced plans to run weekday Dart services along Dublin's east coast every 10 minutes.
But trade unions representing drivers said there was no agreement on the proposals. The more regular service was due to start on April 10.
Irish Rail has blamed "trade union intransigence" for scuppering its planned timetable changes.
Both Siptu and the National Bus and Rail Union represent Dart drivers.
An Irish Rail spokesman said: "We apologise to customers that it has not been possible to introduce the expanded frequency.
"Irish Rail will however increase train size on a range of peak Dart services to address growth in passenger numbers.
"From April 10 we will deliver extra peak capacity for thousands of customers in both morning and evening peak."
Irish Rail alleges that trade unions are using " the requirements of our customers as a bargaining chip in pursuit of completely separate issues" including pay rise claims of up to 25%.
The semi-state company said it has already trained extra drivers for increased frequency on Dart services.
"The trade unions have also damaged the interests of drivers, as the extra drivers who had been trained to allow the service to expand will reduce the need for additional hours from drivers within the existing schedule," the spokesman added.
But Siptu said there are simply not enough drivers or trains.
Union organiser Paul Cullen said Irish Rail claims on the stalled 10-minute service had no basis in fact.
"To run such a service requires at least an additional 20 drivers and these workers will not be in place until June," he said.
"There are also currently not enough train units to operate a 10-minute Dart service in a manner which will actually increase operational capacity.
"What management had proposed was a more frequent service but with less capacity per train."
Siptu said drivers will not take on any extra duties until progress has been made between both sides in an ongoing dispute over workload and an outstanding pay claim.
Irish Rail said existing commuter services will remain unchanged until further notice, bar some minor changes to the Dublin/Belfast Enterprise services from April 10.
NBRU general secretary Dermot O'Leary accused Irish Rail of "peddling untruths".
"Blaming Dart drivers and their trade unions is disingenuous in the extreme. Peddling untruths in relation to the availability of newly-trained drivers is complete and utter fabrication and is designed to deflect away from the fact that management at Irish Rail did not engage in the necessary advance planning for recruitment and negotiations with its staff.
"Refusing to engage with worker representatives on issues in relation to pay at a time when passenger numbers and revenue are heading back to peak levels is neither tenable nor sustainable."
Mr O'Leary repeated his union's warning of a "summer of discontent" on Irish railways unless progress is made on pay claims.