Bus Éireann has begun issuing four weeks' protective notice to staff.
The SIPTU Branch Secretary confirmed that some staff had received their notices today.
Tony Tobin said the company was in the process of issuing notices to the remainder of its 3,500 staff.
The only exceptions are 250 clerical staff who have served strike notice on the company effective from 25 October, in a dispute over pay.
The clerical workers have rejected a Labour Court recommendation, which would have given them a 15% increase subject to certain change provisions.
Their industrial action would consist of the withdrawal of labour on 25 October, Tuesday 29 October and every Monday and Tuesday thereafter until the dispute is resolved.
