SIPTU is accusing Transdev of doing everything it can to escalate the Luas dispute.
It follows the company's warning to Luas drivers that they'll be docked a full day's pay for staging a half-day strike.
Managing director of Transdev Gerry Madden claims the shorter stoppages will cause almost as much disruption as full day strikes.
But SIPTU's Owen Reidy says the company is trying to provoke the drivers.
"What Gerry and his colleagues are doing by taking this unilateral unofficial action, cutting the pay, suspending the sick pay and now this action announced today, is actually adding fuel to the fire making a bad situation worse and I think that is regrettable."