SIPTU claims some drivers were excluded from talks on Monday
The Luas operator has described plans for four more days of strike action by drivers as "a total disrespect to the public".
Drivers say they have served notice of strike action for the 2nd, 3rd, 23rd and 24th of April.
One-day strikes are already scheduled for St Patrick's Day - March 17th - along with Easter Sunday and Easter Monday.
Talks at the Workplace Relations Commission (WRC) aimed at averting further strikes broke down on Monday.
While the head of the WRC, Kieran Mulvey, told Newstalk yesterday groups had been invited back for further talks on Friday.
Drivers are seeking pay hikes of 30% over five years - after they had previously asked for increases of up to 53%.
Luas operator Transdev says it is "extremely disappointed" with SIPTU and Luas drivers - and believes its shows "a total disrespect to the public, Luas customers and business".
Gerry Madden, managing director of Transdev, said: "The public will have no tram service for St Patrick's Day, no tram service for Easter Weekend and now no tram service for two weekends in April".
"The public, Luas customers and business are the real losers here".
"Transdev is in talks tomorrow in the Workplace Relations Commission with three out of the four grades in the company. Luas traffic supervisors, revenue protection officers and supervisors".
"All of these grades are willing to enter talks with the company in the WRC".
"Transdev is willing to talk with the driver grade in the WRC but have stated that those talks will only be meaningful if the pay claim is modified to a reasonable point".
SIPTU divisional organiser Owen Reidy says that tram drivers have been left with no other choice but to strike.