Ambulance personnel belonging to the Psychiatric Nurses Association have commenced their fourth day of strike action over union recognition.
Yesterday, the PNA warned that further strikes were possible if the HSE did not agree to allow it represent its members in the National Ambulance Service.
The HSE says that it already recognises SIPTU and FORSA in the NAS, and that recognising more unions would lead to fragmentation, destabilisation, and impair industrial relations.
The HSE said it was continuing to maintain its contingency arrangements, and that calls were being responded to.
However, it confirmed that Department of Defence ambulances were deployed on a number of occasions during yesterday's 12-hour stoppage.
The HSE said in a statement: "The total number of calls yesterday was 632, all answered in a timely and appropriate manner.
"The army ambulance vehicles were involved in a total of nine calls yesterday and while the air ambulance service was available, there were no call outs.
"The HSE would advise the public that in an emergency situation they should call 112/999 as normal and the National Ambulance Service will respond."
SIPTU, which represents the majority of ambulance personnel, is working normally as it is not in dispute.
PNA General Secretary Peter Hughes yesterday warned that further stoppages cannot be ruled out if the HSE does not agree to recognise the PNA's right to represent its members.
The PNA executive will meet on Tuesday to consider its next move.