IMPACT members in the Limerick group of hospitals have begun industrial action in protest at the €250,000 salary paid to its chief operations manager.
Staff at five hospitals are refusing to report to or cooperate with the manager from this week.
The work-to-rule is taking place at Ennis General Hospital, University Hospital Limerick, Limerick Maternity Hospital, Croom Orthopaedic Hospital and Nenagh General Hospital.
The action was supported in a ballot by 90% of the IMPACT members.
The union says the action will not affect service delivery.
However, it has warned that it could escalate if staff are penalised for refusing to work with the manager.
IMPACT Assistant General Secretary Andy Pike said the action was designed to highlight the opposition of hospital staff to what it says are the excessive and unwarranted salary payments being made through a management consultancy.
He said: "The HSE director general has said the mid-west hospital group needs additional administration staff to provide vital services to patients.
"The money spent employing just one management consultant would cover the costs of at least five clerical staff to help the hospitals cope with increasing demands.
"In these circumstances, staff very much resent reporting to a senior manager who is being paid at least twice the correct rate for the job," he added.
He said that IMPACT is writing to the Public Accounts Committee who are investigating the employment of the management consultant.
The work-to-rule is not expected to affect the delivery of medical services, but the union warns that the action could escalate if any staff are penalised.
There has been no comment so far from the HSE in the region.