Up to 500 ambulance staff are threatening to intensify their campaign of strike action.
They have already had eight days of industrial action and will protest outside Leinster House this afternoon.
It is over the HSE's refusal to allow the Psychiatric Nurses Association to represent them.
Siptu also represents some ambulance staff, and the HSE is refusing to recognise another union.
However Tony Gregg, from the PNA's ambulance branch, says the campaign will continue.
Mr Gregg said: "We finished off before the breaking of the Dáil for the summer recess, we had rolling strikes and if there is no resolution to this we will continue to have rolling strikes and we'll escalate them from the previous 24-hour strikes to further longer duration strikes."
