The NBRU says its members will not be cooperating with any changes "in the absence of an agreed forum"
Bus Éireann has confirmed it is planning a ban on unplanned overtime at the company - along with a range of other cost cutting measures.
Management have written to staff saying they are also proposing the immediate implementation of a recruitment ban and an end to flexi-time arrangements for clerical staff.
Other measures described in the letter sent to staff include efforts to improve fuel efficiency and stricter controls on bus hire.
It comes as the company faces an "uncertain future due to the precarious state of our finances".
The company last week confirmed that it was facing insolvency within 18 months unless “decisive actions” are taken.
Particular concerns having been raised over Expressway services, although the company's CEO has stressed that Expressway will continue to be a part of the company.
The National Bus and Rail Union (NBRU) has described the proposals announced today as "ludicrous", and is instructing members not to cooperate with any changes to work practices.
It wants a forum set up to discuss the crisis at the company.
NBRU General-Secretary Dermot O'Leary said: “Our members at Bus Éireann will not be cooperating with any changes to current terms and conditions or work practices in the absence of an agreed forum being convened to discuss all the issues which have contributed to the crisis at Expressway.
"The notion that tinkering around the edges of staff entitlements will resolve this problem is simply ludicrous and will only serve to further antagonise workers at Bus Éireann."
He added: "There is an obvious attempt being made here to inject volatility into Company/Staff relations to create an impression that workers are being unreasonable and that the totality of the problem, and therefore the solution, lies within normal industrial relations parameters. Nothing could be further from the truth."
In a statement, Bus Éireann say acting chief executive Ray Hernan "outlined to staff a number of cost efficiencies that the company must implement due to the threat of insolvency.
"To achieve cost competitiveness and ensure future sustainability, there will be a root and branch review of all aspects of the business.
"Further savings will be required to put the company on a sound commercial footing, but as a first phase, specific cost saving measures were announced today.
"These include a ban on overtime, stricter control on bus hire, cessation of flexi-time, improved fuel efficiency and a ban on all discretionary spend", it adds.