Luas workers have voted overwhelmingly for industrial action in a move that could see widespread disruption from early next month.
Workers of all grades voted 221-2 in favour in strikes - which are likely to come in the form of one or two-day stoppages.
SIPTU organiser Owen Reidy told the Herald that turnout was 99pc, which he said sends a "very strong message" to the company.
The decision to strike centres on a row over pay, with Luas drivers seeking to be paid in line with their counterparts across the public transport sector.
The drivers insist their pay has been "stagnant" in recent years and is now significantly below that of drivers in Irish Rail.
These demands total €30m over five years, it is understood.
SIPTU members will meet next week to discuss the possible timing and form of the industrial action they will conduct.
In a further statement last night, the union said its members feel their levels of pay are unacceptably low.
"Since it started operation in 2004, the Luas network has grown rapidly, with several extensions added and new developments planned," the statement said.
"Correspondingly, during this period passenger numbers have also consistently increased. Transdev successfully retained the contract to operate the Luas network in September 2014.
"The company has consistently returned annual profits from the operation of Luas.
"Our members believe they are underpaid, particularly when compared with Irish Rail.
"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."