The union says the service has been left "crippled and staff within it demoralised"
The recruitment and retention crisis within mental health nursing services will be discussed by the HSE and the union SIPTU in coming days.
SIPTU says nurse staffing levels within the public health system were reduced by more than 5,000 between 2009 and 2014.
There are currently believed to be more than 600 vacancies within mental health nursing services - and this is set to increase by another 400 due to pending retirements.
Kevin Figgis, SIPTU Sector Organiser, said: “Two tier pay systems, the removal of allowances, no funding for further education, in addition to the continuing effect of the recruitment freeze, have left the service crippled and staff within it demoralised.
“The policy pursued by the HSE in 2015 to attract the return of graduate nurses and midwives to Ireland, with the approval of Government departments, failed to provide any significant return. In fact, mental health nurses agreeable to return under the scheme were in single figures," he added.