The Irish Nurses and Midwives' Organisation has said the Health Service Executive is looking for an extra €80m in savings under the Haddington Road Agreement, on top of €212m this year.
The HSE has said it is simply seeking to implement the terms of the Haddington Road Agreement, which the INMO signed up to.
The HSE said it requires the extra hours identified under the agreement to be used in the first instance to reduce the health sector pay bill as per the intention of the agreement.
The INMO claims under a new plan to save money, the HSE wants to use health care assistants to fill vacancies instead of agency nurses.
From October, it also plans to only replace essential vacant posts with graduate nurses and interns with less experience, the INMO said.
In the disability sector and elderly care sector, it wants to change the 60:40 nurse-support staff ratio to 40:60.
INMO General Secretary Liam Doran said the proposals would seriously compromise patient care.
Qualified nursing staff in the community who leave or retire would be replaced with young graduates of less experience and without peer support, the INMO has warned.
The 80-page plan, presented to health unions at the Haddington Road Oversight Body meeting last week, looks to cut staff levels, the skill mix, and nursing management structures.
It said an extra 520,000 extra nursing hours can be squeezed out of the Haddington Road Agreement.
It also wants to secure more junior hospital doctors from eastern Europe on fixed-term contracts.
Hospitals are being given revised cash savings budgets with the HSE seeking over €56m in savings.
There is to be a 5% cut in agency staff and overtime from June.
The plan states that €4bn has been taken from the health service and 12,000 staff have gone since 2008.
It says the mental health area is seen as offering the greatest potential for generating extra Haddington Road Agreement hours.
The document notes that given recent pay reductions, there is very little difference between a newly-appointed mental health nurse at the bottom of the pay scale, versus a health care assistant.
But it says that by the time a nurse progresses to their third increment, their pay surpasses that of a HCA.
The HSE wants health care assistants to take on other duties such as ordering supplies, escorting patients, washing and dressing patients and doing ward health-and-safety checks.
A spokesperson for the HSE said it is "extremely cognisant of the importance of patient safety and considers it a priority above all else.
"It is not correct to say that the HSE 'is seeking an additional €80m'."
The spokesperson said the HSE is seeking to achieve the original target set for it by the Department of Public Expenditure and Reform of €290m.
This money has already been removed from the HSE's budget, the spokesperson said.
The HSE engaged PA Consulting Group to assess how to maximise cost savings from the health service this year.
That assessment identified that savings of €212m, of the original much higher target, could be delivered this year, with a margin of error on either side.
The INMO is seeking meetings with the three ministers at the Department of Health to discuss its concerns.