Train drivers at Irish Rail have rejected a Labour Court recommendation on pay.
They voted by 83% to 17% to reject a recommendation over pay and productivity measures.
The Labour Court had recommended a 1.15% pay increase to drivers, on top of a 7.5% increase to all employees over three years agreed last month.
It was hoped this would see driver training guaranteed into the future, to allow for an expanded service.
Iarnród Éireann says it "regrets the decision" by SIPTU and NBRU to reject the recommendation.
The company claims non-cooperation with driver mentoring over the past 20 months has resulted in repeated delays to the introduction of an increased DART frequency, and prevented expansion in other services.
It also says this is blocking career progression for trainee drivers.
Irish Rail management says it will "urgently consider" the outcome of the ballot.
SIPTU organiser, Paul Cullen, says: "The result of this ballot demonstrates the depth of dissatisfaction among our members in the driving grade of Irish Rail following the failure of management to deal with their outstanding issues over the last number of years.
"It is also indicative of the shortcomings of management in dealing with industrial relation matters in Irish Rail and the dismissive behaviour displayed by the CEO of Irish Rail following a recent Labour Court recommendation on pay."
NBRU General-Secretary Dermot O’Leary adds: "There has been quite an amount of dissatisfaction among drivers over recent times, not least because of the anti-worker agenda being pursued by the senior management team at Irish Rail.
"The recent debacle around the Labour Court awarded 'goodwill voucher' its restriction to one outlet and the application process for it is systematic of this anti frontline worker approach."