Luas employees have voted overwhelmingly for industrial action after pay demands were not met.
Representatives at Siptu said they would meet with workers next week to discuss when the action will take and whether it will include walkouts.
The tram drivers, inspectors, traffic supervisors and revenue officials voted 99% in favour following a dispute with operators Transdev, with union bosses insisting staff are underpaid.
John Murphy, Siptu organiser, said: " This ballot resulted from a refusal by the management of Transdev, the company which runs the Luas network, to accept our members' claim for improved terms and conditions of employment as part of a new collective agreement."
The union said the Luas network has expanded since it came on track in 2004 with line extensions, increasing passenger numbers and more developments like the cross city link in construction.
Transdev secured the new operating contract in 2014 with Siptu stating it has consistently returned profits.
"Our members believe they are underpaid, particularly when compared with comparable workers in Irish Rail," Mr Murphy said.
"They do not accept that the current contract between Transport Infrastructure Ireland and Transdev, concerning the operation of the Luas network, restricts possible pay increases to the consumer price index, as has been claimed by their employer."