6,000 members of the Psychiatric Nurses' Association will begin an overtime ban today over a dispute with the HSE.
The union says it is in response to a lack of meaningful progress in talks at the Workplace Relations Commission over recruitment and retention issues in mental health services.
Earlier this week, Health Minister Simon Harris said today's action will have a negative impact on the delivery of care for patients.
PNA general secretary Peter Hughes says the lack of effort made by the HSE to address their concerns means they are to blame for today's action.
"There is a huge reliance on agency and overtime and it will curtail services," said Mr Hughes.
"And it will leave some services that staff will not be able to go on duty because they will not have sufficient staff to safely take over units.
"It's going to have an impact but the responsibility lies solely with the HSE who have very, very little effort to address this problem."