Luas passengers face the prospect of serious disruption after employees belonging to SIPTU voted to hold two 48-hour stoppages during February.
The move, which would affect 90,000 passengers a day, marks a serious escalation of their pay dispute in pursuit of increases of up to 53%.
SIPTU said the company had failed to reach an agreement on the workers' terms and conditions of employment.
SIPTU Utilities and Construction Division Organiser, Owen Reidy, said the first stoppage would take place on Thursday 11 and Friday 12 February.
The second is scheduled for Thursday 18 and Friday 19 February.
He said workers were conscious of the inconvenience to customers, but believed they had no option but to take industrial action.
He said that over the last 18 months, Luas operator Transdev, which employers the workers, had failed to offer a pay rise for the next five years.
Transdev has claimed that Luas drivers are seeking pay rises of up to 53.8% in their current dispute.
The company has warned that it cannot afford to concede the claim, which would cost €6 million a year.
Mr Reidy said that in talks over the last 18 months, Transdev has failed to offer a pay increase and its position has consistently been that it will only offer pay rises in line with the Consumer Price Index (CPI).
Mr Reidy said the CPI is a negative figure and this means that "unlike 96% of other companies across the State, Transdev is not prepared to pay any increase in pay in excess of that attached to workers existing conditions of employment".
Earlier, Transdev Managing Director Gerry Madden said that accountancy firm Mazars has verified that the company lost €700,000 last year, with further losses forecast for 2016.
He defended the current pay and conditions of tram drivers, who are effectively seeking parity with heavy rail drivers working for Iarnród Éireann.
Transdev published pay statistics which show that tram drivers with nine years experience currently earn €42,247 - not including a 6.5% annual bonus.
If their 53.8% claim were conceded, their pay would rise to €64,993.
Ticket inspectors with similar experience currently earning €36,434 would see a 20% pay rise to €43,720.
Ticket inspection supervisors, known as revenue protection supervisors, currently on €39,541 with similar experience would see a 28% increase to €50,771.33 by 2017.
Meanwhile supervisors currently on €47,941 want a pay increase of 21% bringing them to €57,941.
Mr Madden noted that the "vast majority" of employees also received a maximum bonus payment of 6.5% of salary averaging at between €2,200 and €3,000 per employee.
He said over 96% of employees achieve the maximum bonus.
Mr Madden urged the union to engage in further negotiations at the Workplace Relations Commission in line with a Labour Court recommendation issued last month and to avoid industrial disruption for passengers.