The HSE has said that the start today of work to rule action by 28,000 members of the public sector union, IMPACT, has not had a direct effect on patient care.
IMPACT members are refusing to cover posts left vacant by the recent recruitment freeze in the health service.
The union says it will continue the action for the long haul to defend services.
However the HSE insists there is no employment embargo currently in place.
The union claims 2,700 posts have been left unfilled because of the restrictions.
The action includes a ban on all non-emergency overtime, out of hours work and co-operation with HSE reforms.
The staff involved include managers and administrators in the HSE. A national lunchtime protest is also planned for 11 June.
The union has said the HSE's freeze is hurting existing services and preventing the implementation of promised improvements in areas like primary care, disability services, mental health services and care for older people.
IMPACT, which also represents therapists, social care workers and other health staff, said its action would minimise the effect on patients but would target the HSE bureaucracy.
It is also threatening other forms of action, including work stoppages.
