Unions at Iarnród Éireann have voted overwhelmingly in favour of industrial action, up to and including strike action, in a dispute over pay.
95% of National Bus and Rail Union members backed industrial action, while 92% of SIPTU members voted in favour.
The two unions will meet on Thursday to discuss their next move.
The dispute is over pay for what is called "past productivity" - or concessions already given by unions.
It is understood that Iarnród Éireann has ruled out any such payment.
SIPTU organiser Paul Cullen said: "The result of this ballot, which saw 92% of drivers vote in favour of industrial action, indicates the level of frustration felt at the company's refusal to honour an agreement it entered into in September 2014.
"Our members still hope that management will step back from the brink and honour the terms of this agreement to negotiate on past productivity produced by drivers."
He added: "Failure to do so will inevitably lead to the disruption of services provided by Irish Rail."
This afternoon the National Bus and Railworkers' Union, released a statement saying its members voted to engage in industrial action "in response to Irish Rail's failure to recognise their (NBRU) contribution over the last number of years."
General Secretary Dermot O’Leary said the union's members voted to engage in industrial action "at the company's refusal to honour the Labour Relations Commission agreement of the 3 September 2014, which provided for discussions on the contributions which train drivers have delivered over the last number of years".
In a statement, Iarnród Éireann said it "remains available to re-engage with drivers' representatives to discuss productivity, and in this context the ballot for industrial action was unnecessary".
It added: "Dialogue is essential to identify verifiable cost savings through defined productivity measures, the benefit of which we have repeatedly stated we would share between the company and employees.
"Any other course of action would renew the threat to the future viability of Iarnród Éireann, job security for all employees, and the services we provide to our customers, in a situation where we continue to incur losses in excess of €1 million per month."
