Unions say talks at the Workplace Relations Commission aimed at preventing industrial action at Luas must include all workers.
Strike action as been announced for St Patrick's Day as well as Easter Sunday and Monday and two further 48-hour stoppages have been announced for next month.
Tram drivers remain outside the talks process as operator Transdev refuses to meet with them.
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SIPTU's Owen Reidy said he welcomed the talks, but that they were incomplete unless all grades are included: