IMPACT's Central Executive Committee meets today to discuss the revised Croke Park Agreement.
Representing more than 63,000 workers, the union's stance is seen as crucial. The leadership is known to support the deal, having been central to the talks.
IMPACT officials admit the proposal will result in loss of earnings for some, but they argue they successfully reduced the severity of some of the Government's proposals.
The meeting comes as opposition to the deal intensifies.
The 24/7 Alliance is to meet later to look at reaching out to other affected unions with the aim of mounting the "strongest campaign possible".
And the country's largest civil service union will discuss later whether to ballot its members on the proposals in the agreement.
Eoin Ronayne, General Secretary of the CPSU, said it is too soon to consider whether industrial action should be pursued.