Luas drivers have voted overwhelmingly to take industrial action in a pay claim seeking increases of over 30%.
Of the 92% of staff who voted, 99% backed the industrial action.
The precise form of industrial action has not yet been decided, but SIPTU said that stoppages were more likely than a work to rule.
However, there is unlikely to be any immediate disruption to services as they will have to serve at least three weeks’ notice of industrial action to their employer Transdev.
The 240 SIPTU members employed by Transdev operating the Luas light rail system are effectively seeking parity with staff on Iarnrod Eireann's heavy rail service.
The maximum salary for a Luas driver is €42,500 for a 39-hour week, while Irish Rail drivers earn around €55,000 for a 43.5-hour week.
The actual increase sought ranges from 30-50% - depends on the employee's grade.
Even if the claim were spread over five years, it remains significantly higher than the current 2% average increase elsewhere in the economy.
Transdev says it cannot meet the claim, which would add €6m (40%) to its total pay bill of €15m.
SIPTU Transport Division Organiser Willie Noone said the ballot outcome indicated the total frustration and anger of workers pursuing decent pay and conditions.
While the form of industrial action has not yet been decided, he said that in light of the huge turnout and support, stoppages were more likely than a work to rule.
The 21-day notice required means those stoppages could occur during the general election campaign.