Management at Bus Éireann is asking trade unions representing workers there to agree to a reduction in overtime for drivers, in order to save €5m a year.
A letter sent yesterday by the company to both the NBRU and SIPTU called for a reduction in overtime levels by an average of three hours per driver each week.
"The drivers' grade is by far the largest group within Bus Éireann and this is where most efficiency savings can be identified and these must be implemented.
"These inefficiencies have resulted in unnecessary overtime and staff employed who are now surplus to our business requirements," the letter said.
The correspondence is part of ongoing attempts to avoid strike action at the semi-state, which has been threatened over plans to reduce costs.
Unions deferred planned strike action in February to allow for talks in the Workplace Relations Commission (WRC), however the talks have so far been unable to find a resolution.
Bus Éireann lost €9.4m last year, and has been trying to secure a total of €30m in savings, with €12m of that from payroll.
In the letter sent yesterday, the company's management said significant cost savings must be achieved and its preference was to do this through agreement with the unions.
And it said it would reduce management head count has part of cost-saving plans.
In a letter of response, SIPTU said it knew a resolution could only be found through negotiations and dialogue assisted by the WRC.
"SIPTU can embrace and commit to improved efficiencies and this commitment comes with the principle that payroll reductions come from agreements and not from any dictates," it said.
The NBRU said the union was already on public record with regards to willingness to engage on an efficiency agenda across all grades.