The company says the decision to delay a 10 minute service is "due to trade union intransigence" - a claim rejected by SIPTU
Irish Rail has delayed plans to introduce more frequent DART services from April 10th.
Drivers with SIPTU and NBRU were threatening strike action over the plans to operate DARTs every 10 minutes.
Last week the unions withdrew from negotiations at the Workplace Relations Commissions (WRC) - saying they will not discuss rosters without a discussion about pay too.
In a statement today, Irish Rail said: "We apologise to customers that it has not been possible to introduce the expanded frequency".
"Iarnród Éireann will however increase train size on a range of peak DART services to address growth in passenger numbers. From 10th April, we will deliver extra peak capacity for thousands of customers in both morning and evening peak," the company adds.
Irish Rail spokesperson, Barry Kenny, says unions have refused to engage in talks to resolve the dispute.
He claims the company has faced "intransigence" from the trade unions.
Kenny also said extra drivers have been trained for the increased frequency, and that productivity is an entirely separate issue.
However, SIPTU has rejected any claims that unions are responsible for the postponement.
In a statement, SIPTU organiser Paul Cullen said: "In reality, the introduction of a 10-minute DART service is an integral part of discussions between the company and trade unions concerning productivity. To run such a service requires at least an additional 20 drivers and these workers will not be in place until June".
“There are also currently not enough train units to operate a 10-minute DART service in a manner which will actually increase operational capacity," he added.
While the National Bus and Railworkers' Union (NBRU) says it "categorically refute(s)" Irish Rail's reasons for shelving the service.
General-Secretary Dermot O'Leary said: "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".
Acting Transport Minister Paschal Donohoe said that he remains committed to the 10 minute DART service despite this setback.