Talks between Luas light-rail management and SIPTU aimed at averting two 48-hour strikes next month have ended without resolution.
It is not yet known whether talks will resume at a later date.
Luas staff are seeking pay rises of up to 53% which they say would bring them into line with heavy-rail drivers at Iarnród Éireann.
However, their employer Transdev, which has the contract to operate the Luas, said it cannot afford to pay the claim which would add €6m to its annual payroll bill, particularly as it lost €700,000 last year.
As he left the talks after three hours of negotiations, SIPTU official Willie Noone said that not only had no progress been made, but the offer from the company that had previously been on the table had been withdrawn.
He said the industrial action was still on and the situation was "all negative".
Mr Noone rejected suggestions that the workers had pitched the claim too high.
He said it would be up to the Workplace Relations Commission (WRC) to decide whether talks would reconvene at a later date.
SIPTU'S John Murphy said: "Our members are very disappointed at the approach of management to the talks at the WRC this morning.
“The company failed to show any initiative in the effort to find an agreed resolution to the dispute,” he added.
Transdev Managing Director Gerry Madden said SIPTU had not made a "substantial" reduction in their claim - but denied talks had broken down.
He said they awaited further instruction and looked forward to coming back to the WRC hopefully at some time soon to find a resolution.
If the strikes proceed on 11, 12, 18 and 19 February, they will disrupt travel for 90,000 passengers a day.
Meanwhile, DART drivers belonging to the National Bus and Rail Union have voted overwhelmingly to take industrial action up to and including strike action if Irish Rail proceeds with its current plans for a ten-minute frequency timetable for the service.
NBRU General Secretary Dermot O'Leary confirmed that 99% of drivers had voted, with all of them backing industrial action.
The result of SIPTU's ballot is not yet known.
The ten-minute service had been due for implementation on 31 January.
However, yesterday the company announced that it was deferring the new arrangements until 10 April to allow for consideration of submissions to a public consultation process and to finalise arrangements for services on St Patrick's weekend and Easter.
It said it also needed to finalise how the ten-minute frequency would affect the enterprise service from Northern Ireland.
Mr O'Leary urged Iarnród Éireann to cooperate with the unions to expedite the issues on past and future productivity within the process previously recommended by the Labour Court.
He said the company's steadfast refusal to date to pursue this route appeared to indicate that the company was intent on forcing unnecessary conflict, which would contribute to even lower morale among the workforce and inconvenience its own passengers.