Bus Éireann has informed staff that it will implement cuts to terms and conditions from Monday 20 February, which result in pay cuts of around 10%.
Unions have described the company's move as an engagement in warfare, saying they are prepared for the battle.
In today's letter to staff, Acting Chief Executive Officer Ray Hernan says that from 20 February, the Sunday premium will be reduced from 100% (double time) to 20%.
He says rota and shift premium payments will cease.
Overtime rates will be reduced to time and a quarter, with double time for public holidays.
There will be a general 10% reduction in allowances across all grades, and in future, sick pay benefits will be based on basic salary only.
From 20 February, the company will also be introducing a new more flexible method for covering core drivers, which may include the employment of part time and casual staff.
He estimates the effect of the proposed changes on average earnings will be approximately 10%, after an average 2% increase offered on basic pay.
He says that to minimise the impact on payroll, he is targeting the maximum number of savings on non-payroll items.
In apparent anticipation of a robust response from unions, Mr Hernan warns that any industrial action will result in a significant deterioration in the company finances, meaning even greater savings would have to be found.
He says the company has delayed implementation until 20 February in order to facilitate meaningful engagement with staff representatives.
However, he warns that implementation of measures after that date will be critical to safeguarding the maximum number of viable jobs and avoiding the risk of insolvency.
He says that a review of all Bus Éireann structures will be complete by the end of March.
He concludes by urging unions to reconsider their position and engage with the company in a meaningful way to secure the future of the company.
In a press release, the Bus Éireann Trade Union Group described the move by Bus Éireann as "draconian rules of engagement."
The group said that the company is "determined to continue with providing cover to allow the Shareholder to abdicate its responsibility by turning the focus on ordinary decent Bus Workers to fix a problem not of their creation.
"The notion that a State-owned Company would be instructed to engage in open warfare with its own staff by attacking their core terms and conditions of employment is nothing short of appalling and will achieve absolutely nothing in terms of solving this policy induced crisis."
The union group also said that they are "fully prepared to engage in a battle to protect its members and this most critical of public services that is relied upon by so many of our citizens right across this country".
Minister for Transport Shane Ross has repeated that he cannot intervene in the dispute at Bus Éireann.
He said it would not be in the interests of the taxpayer or the rural communities if he did so.
Mr Ross said any intervention would lead to an expectation that he would produce a cheque book to solve the dispute and he is unable to do this.