The Psychiatric Nurses' Association said it has concerns about the manner in which acute inpatient beds will be closed at St Bridget's Hospital in Ballinasloe, Co Galway.
The HSE has announced that 22 beds will close by the end of the year.
Resources will be "re-focused" on community based care to help implement the Vision for Change plan.
PNA National Secretary Noel Giblin said it was essential that established community care teams were in place before the acute unit closed.
He said it was widely accepted that these teams could lead to enhanced services, but he raised questions as to the timeframe for the move.
Mr Giblin said a community care dynamic must develop over time, to ensure medical staff interact in a way that brings about the best level of care to patients.
He said he hopes the bed closures will not proceed until care teams in the area are fully up and running.
The HSE said community mental health teams will be developed and strengthened to provide care and treatment nearer to service users' homes.
There will be 35 acute inpatient beds that are maintained in Galway city and 22 acute inpatient beds will be in Roscommon.
The HSE said there will be no impact for current inpatients in Ballinasloe because the bed reduction will take place on a phased basis.
Galway East Fianna Fáil TD Micheál Kitt has also questioned the decision. He said early intervention for people with mental health issues is central to provision of care.
Mr Kitt said a "patient-first" approach would be better served by maintaining access to local services that are currently being provided at St Bridget's Hospital.
He said he is fully in favour of the Vision for Change plan, but he has questioned the way it is being interpreted by the HSE.