Army paramedics will be drafted in today as 500 ambulance staff strike in the ongoing row over the right to trade union representation.
Members of the National Ambulance Service Representative Association (Nasra) are walking off the job from 7am to 5pm.
They want the HSE to deduct their union subscriptions from their pay.
But the HSE is refusing on the grounds that "recognition of other associations or unions would undermine the positive engagement that exists and would impair good industrial relations in the National Ambulance Service".
Contingency arrangements are in place to ensure a service is in place today and Army paramedics will be called on to support the service.
HSE ambulance service managers are also to take on frontline duties.
Siptu, which represents most staff, has confirmed that it expects its members to work as normal.
Members of Nasra will also strike on Thursday, February 28, and Friday, March 1.
The Psychiatric Nurses Association (PNA), which incorporates Nasra, has again called on the HSE to avoid further escalation in this dispute and engage with the industrial relations machinery to allow members be represented by the trade union of their choice.
The HSE said the principle of engaging only with recognised trade unions had been acknowledged previously by the Labour Court in a dispute involving the PNA and a different public-sector employer.
"With this in mind, National Ambulance Service will stand by the agreements that it has made with recognised unions and will not undermine those agreements by engaging with other associations or unions. This approach is in keeping within Government policy and supports the consolidation that is happening within the wider trade union environment," it said.
The focus for the National Ambulance Service in all circumstances was to ensure that service and care delivery was not compromised in any manner and a contingency plan is in place, the HSE said.