A third trade union has voted for industrial action at Bus Eireann over a plan to slash employees' pay and expenses.
Siptu members voted 91% in favour of stoppages as the transport firm lines up to enforce its cost-cutting programme from Sunday.
The trade union, which has 900 members at the bus company, joins the National Bus and Rail Union and the Transport and Salaried Staff Association in voting for industrial action.
Bus Eireann has claimed that failure to agree reductions in overtime and expenses will lead to losses of 16 million euro a year.
Willie Noone, Siptu organiser, warned of a widening of the dispute to other state-controlled transport companies.
"Our members have always been willing to change working conditions and practices within reason but are not prepared to have changes unilaterally imposed on them," he said. "This decision is a direct response to the threat by management to dictate change and cut the pay of our members."
Strike action is expected to begin next Thursday, barring an intervention by industrial relations trouble-shooters. Siptu said members voted to strike due to the seriousness of the issues as other action was considered less effective.
The union said it expected services to be hit nationwide, and possibly hit other companies in the CIE group.
"If the company proceeds on its current course it is likely that workers across the CIE group will be forced to take similar action," Mr Noone said.
"The attempt by the company to have all issues referred to the Labour Court was seriously flawed and was a crude attempt to fast track the imposition of changes without adhering to agreed procedures."