Around 500 ambulance personnel belonging to the Psychiatric Nurses Association are to escalate their industrial action with an overtime ban next month in a dispute over union subscriptions.
From 7 November, staff in question will refuse to make themselves available for additional shifts outside their rostered hours.
As yet it is unclear what impact this might have on patients.
Traditionally, SIPTU represented the 1,800 ambulance personnel, but around 500 have chosen to transfer to the National Ambulance Service Representative Association (NASRA), an affiliate of the PNA.
However, the Health Service Executive has refused to deduct their union subscriptions through the payroll system and remit them to the PNA/NASRA because it says SIPTU is the only union with negotiating rights for the grade.
Last week, the PNA/NASRA personnel commenced a campaign of industrial action involving a refusal to work with vehicles did not comply fully with regulations.
They have also been refusing to use any of their personal equipment or technology for work purposes unless they are receiving an allowance for doing so.
In addition, staff who are not in possession of a full uniform and Personal Protection Equipment, which is free from contamination, are standing down - and have only commenced their duty at their official roster time.
A previous planned strike was deferred after the HSE threatened to seek a court injunction to prevent it.
The union says it remains available for talks about contingency arrangements and a potential resolution of the dispute.
The HSE said the National Ambulance Service could not comment on the threatened escalation of industrial action at this time.