The board of Bus Éireann will decide its next move at a meeting this morning as it faces the prospect of an all-out strike and the collapse of the company.
Sources revealed it had set a deadline of a board meeting on March 27 to sign off on a €30m cost-cutting plan to secure its survival.
Talks with unions on payroll cuts failed earlier this week for the second time.
Sources blamed militant shop stewards at garages including Cork and Limerick for refusing to agree to any changes to their overtime arrangements to cut the yearly €13m bill, which is a tenth of payroll costs.
The company wants drivers to be available to work five out of seven days a week. Entering talks, Siptu and the NBRU had said they were willing to discuss efficiencies.
But Siptu said Bus Éireann management's "intransigence" had led to a situation where strike action was unavoidable.
It said its members would never try to justify unnecessary overtime.
"However, our representatives will not allow for wages earned in excess of a basic rostered week, that brings a meagre wage to a decent wage, to be removed without agreement by management," it said.
It said management proposed that some drivers would drive considerably more hours and retain traditional payments, but others would not be given the same opportunity and their pay would be drastically cut.
Meanwhile last night, Fianna Fáil accused Transport Minister Shane Ross of "reckless behaviour".
The party's transport spokesperson Robert Troy told the Irish Independent that Mr Ross's "hands-off approach" in relation to the pending strike was not acceptable.
"The minister's excuse that he cannot get involved in this strike does not wash with the public," Mr Troy said.
The Fianna Fáil TD added that any such strike could drive Bus Éireann users away from using bus services.
"A strike is not good for anyone - workers or the public - but Mr Ross has shown no desire to sort this out.
"He may as well be away for St Patrick's Day, because he is doing nothing to resolve this dispute."
Mr Ross's spokeswoman did not respond to a request for comment last night.
When pressed via text message over the issue, the spokeswoman said she would respond "tomorrow".