The National Bus and Rail Union has cast doubt on whether the ten-minute DART service proposed by Minister for Transport Paschal Donohoe for early next year will take effect.
NBRU General Secretary Dermot O'Leary said the issue of a ten-minute frequency service had been raised by management during recent discussions at the Workplace Relations Commission, though no agreement was reached.
The Labour Court subsequently recommended that Iarnród Éireann and its unions should enter further discussions on productivity and that they should engage in a benchmarking exercise on working hours in comparable operations in the UK and Northern Ireland.
That work was to be completed within four months.
He said the union was currently balloting its members on the Labour Court recommendation. The ballot will be completed by 14 December.
Mr O'Leary said that from the point of view of train drivers, the proposed ten-minute DART service would be "productivity-based".
He added that it would be a matter for members working on the DART to decide if they would be in a position to cooperate with any proposed changes.
He said they would be particularly concerned about how such a service could affect the current rostering regime, and the adequacy of the resource pool of drivers within the DART.
Iarnród Éireann said the ten-minute service was completely separate from productivity issues, as the company is training new drivers so that the overall service can be delivered within existing arrangements.