SIPTU and NBRU members at Iarnród Éireann have voted overwhelmingly in favour of industrial action in a dispute over pay.
SIPTU, which represents 1,900 workers at the company, voted by 84% in favour of industrial action.
Rail passengers may face disruption in the run-up to Christmas after talks aimed at averting strike action at Iarnród Éireann concluded without agreement last night.
Both sides took part in more than 12 hours of talks at the Workplace Relations Commission yesterday. An agreement over a pay deal was said to be close, but talks collapsed at the last minute.
Management at Iarnród Éireann accused the unions of rejecting a deal offering 1.75% for one year with further improvements possible subject to productivity.
However, furious union representatives accused Iarnród Éireann Chief Executive David Franks, who was not at the talks, of collapsing discussions at the eleventh hour.
Unions are now expected to announce a campaign of unrest commencing on the October Bank Holiday weekend, causing disruption for tens of thousands of customers.
Union representatives are to meet this afternoon to agree on a date for the action.
NBRU General Secretary Dermot O'Leary blamed Mr Franks and Minister for Transport Shane Ross for the dispute.
Speaking on RTÉ's Today with Sean O'Rourke, Mr O'Leary said that Mr Franks has questions to answer in relation to "his stewardship" and accused him of "interference" in the negotiating process.
He said that union officials were confident that they were close to a proposal emerging last night, but that the "rug was pulled from under our feet and WRC feet".
In a statement, Mr O'Leary added: "We will meet with trade union colleagues this afternoon in order to agree a coordinated strategy of industrial action over the coming weeks and the run-in to Christmas.
"It is long since passed time that political leadership was provided by the Minister for Transport with regard to the publicly-owned public transport companies."
SIPTU Divisional Organiser Greg Ennis said industrial action would be regrettable, but the unions were strong and united.
Barry Kenny, corporate communications manager with Iarnród Éireann, said that progress had been made in the talks, but said that the NBRU was mistaken to think it was "extraordinarily close" to a proposal.
He said that all sides should return to the Labour Court to resolve outstanding issues and that the unions should not be threatening distruption to rail services.
Iarnród Éireann warned that industrial action would only worsen the company's financial position and cause uncertainty to passengers.