The HSE has been ordered to pay a fine of €10,000 and costs of €13,000 after nurses were injured in an assault by a patient in Co Meath.
The HSE pleaded guilty to four health and safety breaches at Trim Circuit Court.
It follows an investigation by the Health and Safety Authority into a "serious incident of violence and aggression" where a number of nurses were assaulted by a patient at Our Lady's Hospital in Navan, Co Meath.
Nursing staff suffered injuries during the incident at the Department of Psychiatry on 29 August, 2016.
There was no emergency response team to respond to the activation of employee's personal alarms when they were unable to contain violence and aggression and violence, according to the HSA.
The Authority said there were no swipe card-operated magnetic doors to segregate and contain high-risk patients and there was no security guard.
The HSE pleaded guilty that there was no adequate "assessment, documenting, communicating and management of risks" to the safety, health and welfare of employees in the Navan Unit arising from the known hazard of violence and aggression from high-risk patients.
The HSE also pleaded guilty to failing to manage and conduct work activities, specifically the operation of the Psychiatric Unit, in such a way as to ensure, so far as was reasonably practicable, the safety, health and welfare at work of employees.
"Where there is a known risk of violence and aggression in a workplace, employers have a duty to ensure that the appropriate precautions are in place and staff are protected," Mark Cullen, Assistant Chief Executive of the Health and Safety Authority, said.
"Failure by the HSE to do so in this case led to serious injuries for the staff concerned." he said.