Siptu has warned the Health Service Executive (HSE) that any attempt to change the terms and conditions of its staff would be met with "full-scale industrial action" .
Siptu national industrial secretary Matt Merrigan accused the HSE of disregarding social partnership and engaging in "management by dictat" following its recent decision to extend a recruitment freeze.
In an address to the union' conference in Tralee today, Mr Merrigan said the economic climate meant the health service was facing further spending cuts next year.
He predicted that the public spending on health would rise by some 4.5 per cent next year but would fall by 3 per cent in real terms because of rises in the level of medical inflation and the pay bill.
Mr Merrigan said if there were any serious attempts by HSE management to take away the hard-won rights of workers that had been established in partnership agreements, the union would support "any action up to and including full-scale industrial action in respect of dealing with these issues.
"We are not going to be rolled over once again by the employers who want to dictate the way health services are run," he added.
On Monday, the HSE decided to continue with its recruitment freeze, which began last month, in a bid to reduce its financial deficit, which is running at over €200 million.
But the unions - which include Siptu, Impact, the Irish Nurses' Organisation and the Irish Medical Organisation - believe that the HSE was in breach of the "Towards 2016 pay" agreement when it announced the staffing restrictions.
Those unions represent over 100,000 health sector staff.
The HSE has come in for severe criticism this week including from Cabinet minister Eamon Ó Cúiv and former Minister for Health Mary O'Rourke.
Both said the service appeared overly bureaucratic.
Minister for Health Mary Harney today acknowledged "there are quite a number of people who are unhappy with the HSE" but maintained it was doing its job well.
And she reiterated that the HSE would not receive more funding this year meaning the stand-off with unions is likely to continue.