Aer Lingus pilots, whose pay has been halved due to the Covid-19 emergency, will see earnings gradually recover to 80% of normal by April if they accept proposals in a ballot by their union Fórsa/IALPA.
However, SIPTU said it was "unacceptable" that the pilots had been given time to hold a ballot on their crisis proposals, while non-pilot unions had been denied that right.
Under the pilots' deal, in return for certain changes they will see their pay - currently halved like the rest of the workforce - rise to 60% of pre-coronavirus rates in October.
In January it will increase to 70%, and in April it will hit 80%.
As with other Aer Lingus grades, where staff work for less than half their hours due to a lack of business, any "overpayments" will be recouped when the airline recovers.
It is understood that one pilot briefing document stated: "Once pay increases, it cannot decrease even if work drops off."
The Covid Crisis Recovery Plan for ground operations, maintenance and cabin crew finalised on Friday would have preserved pay at 50% (even where activity fell below that level) and averted around 200 planned lay-offs in return for implementation of certain work practice reforms.
Any "overpayments" would have been recouped when the business recovered, and reward or "recognition" payments would also be negotiated at that stage.
The company also reserved the right to "re-visit" these measures should the situation "materially deteriorate".
Aer Lingus set a deadline of Monday evening for acceptance, which left no time to ballot members.
When Fórsa insisted on balloting cabin crew on the proposals, the deadline was not met and management withdrew the Recovery Plan proposals.
The company is now preparing to implement not just the additional pay cuts (to 30% of normal) and layoffs, but also the contested work practice changes.
However, sources suggested the reason the pilots had more time to ballot their members was because they operate to a different monthly payroll system.
In a statement, Aer Lingus said that from early on in the management-union discussions, all parties had accepted that public health restrictions would cause the ICTU Group of Unions to have difficulties consulting with and balloting their members in the normal way.
"It was therefore accepted by all parties that the discussion be structured in the form of a document that could be accepted by the Unions rather than requiring a balloting process in the traditional way," the company said.
"IALPA informed Aer Lingus at the outset of discussions that it had the capacity to undertake remote consultation and balloting processes. Given this and that discussions with IALPA also clearly impacted on express terms of some existing collective agreements, discussions were structured to produce an agreement to be put to ballot," it concluded.
SIPTU had been prepared to accept the deal without a ballot due to pressure of time, public health concerns, and to avoid further pay cuts and job losses.
However, Transport Divisional Organiser Karan O'Loughlin said it was unacceptable and unfair that the company would give one group more time to go through the democratic process, while putting other workers under the pressure of what now turned out to be an "arbitrary" deadline.
She urged Aer Lingus to halt the lay-offs planned for next week and re-engage immediately to ensure that workers have the required protections.
Fórsa's ballot of cabin crew members on the Covid Crisis Recovery Plan is proceeding despite the fact that management has withdrawn it - and will conclude this Monday.