Unite Regional Secretary Jimmy Kelly has said rejection of the Haddington Road Agreement could lead to widespread strikes.
Speaking on RTÉ's Today with Pat Kenny, Mr Kelly said his union will be explaining the consequences of voting No to its members.
He said the implications could include industrial action.
"We've got to give our members the best leadership and guidance that we can give. The consequences of voting no ... it could possibly mean strikes all over the place," Mr Kelly said.
Speaking on the same programme, National Organiser for SIPTU's health division Paul Bell rejected suggestions that the union's leadership has lost credibility.
"We don't believe that there is any credibility issue at all and our general officers Joe Flynn and Patricia King have led our union in extremely difficult times.
"No matter which side of the argument you are on, the fact of life is that the Government are intent on recovering €420m from the public health savings budget and that is a difficult thing to be doing."
Meanwhile, IMPACT has decided to accept the terms of the Haddington Road Agreement on public sector pay and conditions.
The union will not be re-balloting members, as it says the Haddington Road deal contains improvements to the Croke Park II proposals, which the union membership had already accepted.
In a statement, the union said that the agreement would exempt public service employees who sign up to it from what it called "draconian" cuts in pay and working conditions contained in the Government's Financial Emergency Measures in the Public Interest Act which was passed last night.
The so-called FEMPI legislation provides for a three year increment freeze at all pay levels, as well as pay cuts for those earning over €65,000.
It also gives government ministers extensive powers to change employees' terms and conditions, apart from core pay.
However, public servants can avoid the harsher elements of the FEMPI legislation by signing up to a collective agreement.
An IMPACT spokesperson said that the collective agreement would also protect members in other areas including compulsory redundancies, more extensive redeployment and outsourcing.