Strikes by train drivers could threaten the future of the country's loss-making rail system, transport chiefs have warned.
Trade unions Siptu and the National Bus and Rail Union (NBRU) have voted overwhelmingly in favour of industrial action at Irish Rail.
They are locked in a dispute with bosses about being paid for carrying out previous cost-saving measures.
Union bosses will meet on Thursday to decide what action they will order.
Paul Cullen, Siptu organiser, said there is a high level of frustration among members at the company's "refusal to honour" an agreement reached in September last year.
"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 said.
The NBRU warned the fall-out would "inevitably result in service disruption".
Irish Rail said there is no need for industrial action as its managers remain open for talks.
"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," a spokesman said.
"Any other course of action would renew the threat to the future viability of Iarnrod Eireann, 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 euro per month."