The NBRU and SIPTU have said DART drivers will be balloted on industrial action over Iarnród Éireann's planned implementation of a ten-minute DART frequency without the agreement of the union's members.
NBRU General Secretary Dermot O'Leary said Iarnród Éireann informed trade unions at the Workplace Relations Commission on 13 January that they were going to move ahead and introduce the ten-minute Dart service without the agreement of drivers.
"This has left us with no option but to ballot our members in response to what would be a major departure from how we would normally address fundamental change at Iarnród Éireann," said Mr O’Leary.
He added that there may be potential problems with having to reduce DART trains from eight coaches to six or four coaches to facilitate extra trains, which could have a knock-on effect on capacity at peak times.
SIPTU Organiser Paul Cullen said: "Our members are deeply disappointed that the management of Irish Rail seems intent on provoking further conflict with workers.
"This is despite the Labour Court recently proposing a negotiation process which aims to create a forum were solutions can be found to productivity issues at the company," added Mr Cullen.
Iarnród Éireann previously stated 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.
The ballots are expected to be completed next week.