The union says it will immediately move to ballot for industrial action
Pay talks between Irish Rail and the National Bus and Rail Union (NBRU) have broken down without a deal.
Discussions had been ongoing at the Workplace Relations Commission (WRC) in Dublin.
NBRU members say they are "furious" at Irish Rail's "contemptuous attitude towards its staff" in refusing to offer a 'no strings attached' pay rise after a decade long pay freeze.
Unions are calling for a wage increase of almost 4%.
However, Iarnród Éireann say it has accumulated debts of €160m.
NBRU General-Secretary Dermot O'Leary says: "After a decade long pay hiatus, the expectation of a long overdue pay rise for Irish Rail staff has not materialised.
"Our members have looked at colleagues in the public transport sector enjoying pay increases of up to 3.75%, while Irish Rail is thumbing its nose at its own staff by making a pitiful offer, at a time when passenger numbers and revenue at Irish Rail are at a historical high, is contemptuous to say the least."
"The patience of a beleaguered workforce has run out after today's charades at the WRC".
The union says it will immediately move to ballot all members for industrial action up to and including all-out strike.
It says this could see travel chaos for commuters "in the next number of weeks."