The Psychiatric Nurses Association is to ballot members at Galway University Hospital on industrial action.
The move follows a protest by the PNA over staffing levels at the hospital's acute psychiatric unit.
The association contends that the 45-bed unit is not adequately staffed and that this is putting patients at risk.
It said its members cannot provide proper care and there have been repeated breaches of a staffing plan that was agreed with hospital management.
The Health Service Executive said staff have been redeployed to the acute unit and new nursing posts have been approved.
The HSE said staffing levels in the acute psychiatric unit at GUH are not static and fluctuate on a weekly basis, subject to various criteria.
These include the number of patients in the unit, the nature of their illness and their treatment requirements.
However, PNA General Secretary Des Kavanagh said there is no indication that commitments to recruit extra staff for the unit will be met in the short term.
He has called on the HSE to set out in clear terms how it plans to speed up the recruitment process.
Mr Kavanagh said it could take up to three months to hire new staff, due to the legal requirements for prospective employees to be vetted and cleared to work.
Under an agreed staffing arrangement up to 15 nurses should be rostered for duty on each shift.
This is to cover general care and to facilitate the need for one-to-one patient care, based on clinical decisions.
Mr Kavanagh said the staffing complement regularly falls to around ten nurses.
The ballot on industrial action will begin on Monday.