Union members at Irish Rail have voted 'overwhelmingly' in favour of industrial action.
SIPTU members voted by 84% in favour of action amid the ongoing pay dispute at the transport company, while NBRU members backed industrial action by 93%.
The ballot counts come only hours after talks broke down at the Workplace Relations Commission (WRC) overnight.
SIPTU's Greg Ennis said: “It was always only as a last resort that our 1,900 members in Irish Rail said that they would initiate industrial action. However, due to the intransigent and combative attitude displayed by management over recent weeks, they have no alternative."
NBRU General Secretary Dermot O’Leary added: “It would appear that this company is hell-bent on forcing its own staff onto the picket lines and creating an environment which will bring nothing but misery to thousands of rail commuters.”
Tens of thousands of commuters could face disruption over the coming months if strike action proceeds.
Unions will meet later this afternoon to discuss their next step after turning down a possible pay hike at the WRC.
Management say they offered a 1.75% pay rise, based on new productivity measures.
However, unions rejected the deal, with Irish Rail workers saying they still have not been offered a 'no-strings' attached deal.
The unions will now "proceed to conclude and count their respective ballots".
Mr O'Leary says unions will meet at 2.30 this afternoon before announcing the next course of action.
He suggested talks had been "inching towards" a potential pay deal last night.
However, he claims an offer was pulled by 'people that were not in the building' last night - causing the talks to collapse without agreement.
Barry Kenny of Irish Rail, meanwhile, says the dispute needs to be referred back to the Labour Court.
Speaking about his company's offer, he suggested: "We had improved our position - we had offered 1.75% increase for one year to trade unions, and to discuss further productivity issues to fund further improvements beyond that."
He urged trade unions to complete the process and "refer the outstanding issues to the Labour Court, rather than balloting for or threatening industrial action."