Up to 500 ambulance staff are to stage a 24-hour strike tomorrow.
The strike is due to a long-running row over trade union representation rights and the deduction of union subscriptions at source.
Members of the National Ambulance Service Representative Association said they will only deal with calls for life-threatening and potentially life-threatening emergencies.
They are part of the Psychiatric Nurses Association (PNA).
Ambulance staff represented by the trade union Siptu are not involved.
A quarter of all ambulance personnel will not be available for a twenty four hour period.
Pickets are due to be placed at ambulance depots across the country from 7am tomorrow.
The HSE has yet to reveal contingency plans.
It said that the PNA does not have negotiating rights.