A group representing 70,000 frontline workers has warned that firefighters and nurses may not provide emergency cover during a strike if government cuts go ahead.
Normally unions follow a code of conduct that means patients are not put in danger and a skeleton service is in place.
But one of the leaders of the 24/7 Frontline Services Alliance, Seamus Murphy, said the group is giving serious consideration to the threat to pull emergency cover made by one of its members at a rally this week.
Mr Murphy was commenting after John Kidd, of the Irish Fire and Emergency Services Association, warned firefighters would not provide emergency cover in the event of a dispute over cuts to premium pay.
"I think John Kidd is right," said Mr Murphy, deputy general secretary of the Psychiatric Nurses' Association.
"We are entering a phase of industrial dispute in the public service that we've never seen before. We are not going to be made a fool of a second time."
He said he would be surprised if the unions in the 24/7 Alliance provided emergency cover if there are work stoppages.
"We would have to find some other way of dealing with the very, very sick, but every hospital ward would not be kept open like the last time," he said.
"When we provided this cover in the past, in a dispute in 1999, we were black-guarded."
He said the HSE would be given adequate notice.
Meanwhile, union bosses negotiating a new Croke Park deal are furious at fellow unions for accusing them of being "in cahoots" with the Government at a Dublin rally.
Nursing union boss Liam Doran, in particular, has come under fire for heading a frontline workers' campaign against the Government's payroll cuts while staying at the talks.
Unions said they are being "hammered" by their members for remaining at the table while frontline unions are threatening industrial action over cuts to their members' premium pay.
Their anger grew yesterday after 24/7 Frontline Services Alliance's Mr Murphy condemned ICTU for agreeing to enter talks on a deal when the existing one has not run out.
"What Seamus Murphy said about the Congress unions is appalling stuff," one senior union official said. "And Liam Doran needs to get off the fence.
"These are groups that wouldn't support the Croke Park deal in the first place, and are now using its guarantees to defend their pay."
IMPACT, which has over 63,500 members, said virtually no 'uniformed' public servants will be affected by any cuts to higher-paid public servants.
It said the Government's demand for €170m of cuts in Saturday, Sunday and evening premium payments over the next three years, which is the frontline workers' main gripe, could be negotiated.
The union said "larger or equivalent sums" are being targeted in other areas.
It said other categories of staff are facing cuts of the same order as the premium pay 'ask'.
The Irish Nurses and Midwives Organisation confirmed that it is continuing at the talks, but said it is "steadfast" in its position that it will accept "absolutely no cuts to premium pay".
A spokesperson said members wanted the union to stay at the talks so others would not "call the shots".