SIPTU has accused Transdev of "pouring fuel on the fire".
It comes as Luas drivers have been warned they'll be docked a full day's pay during their 4 hour strikes.
Drivers will walk off the job again this week in their long-running dispute over pay - with another six stoppages planned for next month.
SIPTU claims the company, Transdev, is developing a new initiative each day.
Divisional Organiser, Owen Reidy, has said the union is willing to re-enter talks with the company.
“We would accept an invitation from any other agreed third party of course including the labour court.
“We’ve tried to use our own initiative by discussing it with Transdev directly.
“We hammered out a deal with the company directly with three of the four grades so we were optimistic and hopeful that it could be done with the drivers.
“I think with a bit of creativity, a bit of compromise on all sides, including our own it could be done but when we have an initiative every day where the company is cutting people’s pay it doesn’t actually help the situation.”