Around 4,500 psychiatric nurses around the country have begun industrial action over staffing and pay.
The Psychiatric Nurses Association said there will be about 1,000 staff vacancies in the system by the end of the year, and that its members are angry at a lack of recruitment.
The nurses are engaging in a work-to-rule after 87% of PNA members voted for industrial action.
The action includes non-co-operation with clerical and admin duties.
Peter Hughes of the PNA said the action would actually benefit their patients.
"The impact (of the work-to-rule) will release nurses to spend more time doing clinical duties and attending to patients, as they will not be answering phones and delivering bloods etc," he said.