The Psychiatric Nurses' Association has claimed that mental health services are in 'freefall', with 20% of psychiatric nurses retiring from the HSE in the last two years.
In a report to Minister for Health Mary Harney, the PNA says 596 psychiatric nurses retired last year, many due to stress, assaults from patients and cuts in services.
The PNA estimates that up to 250 psychiatric nurses will retire this year.
PNA General Secretary Des Kavanagh said that because of the shortage of nurses and the threat posed by some patients, security companies are being employed in some hospitals to provide secure 'care'.
Mr Kavanagh said that gardaí in riot gear have been called to assist in some acute psychiatric units and secure facilities with increasing frequency in recent years.
He said that the non-replacement of nurses who are retiring early has resulted in a large number of posts remaining vacant and that using overtime and agency staff is an expensive option to fill the gaps.
The PNA said patients suffering from depression, anxiety and bi-polar disorder are having their recovery threatened by their 'frightening and threatening hospital environment'.
The union said mental health services now receive the lowest ever spend as a percentage of the health budget.
According to the HSE, there is an exemption within the staff recruitment moratorium, which should see the appointment of around an extra 100 psychiatric nurses this year.
It also said the apparent increase in incidents may be down to a better reporting system.