Bus passengers face the prospect of significant disruption on 1 May, the Friday of the bank holiday weekend.
It comes after the National Bus and Rail Union voted to hold a day of industrial action over Government plans to put 10% of bus routes out to private tender.
Unions fear that the opening of 10% of bus routes to private operators will drive down pay and conditions.
The proposed industrial action has been strongly criticised by Dublin Bus, Bus Éireann and the National Transport Authority.
Last week, SIPTU drivers at the two companies voted overwhelmingly to back strike action over Government plans to open 10% of bus routes to private tender.
Today, shop stewards decided that if their concerns are not adequately addressed in the next ten days, they would consider issuing formal notice of "substantial" industrial action on 24 April.
The National Bus and Rail Union went further, announcing a day of protest action on 1 May.
General Secretary Dermot O'Leary said the central element would be marches in Dublin, Cork, Limerick Galway and Waterford.
#Nbru vote for strike action at Dublin bus/bus Eireann over potential privatisation of bus routes by over 90% - day of action in 1 may— Ingrid Miley (@ingridmileyRTE) April 13, 2015
Speaking on RTÉ's Drivetime, he said that the union is open to private sector tendering, but said that 10% is too much, describing it as the "thin end of the wedge".
He said the NTA needs to stop the tendering process and negotiate all of the NBRU's concerns.
The union has no assurances that CIÉ companies will exist beyond 2019, he added.
Mr O'Leary apologised to bus commuters for the disruption, but said that the early morning and evening commute should be okay.
There are no plans to extend the industrial action to the rail network.
Govt committed to tendering routes
Meanwhile, the Minister for Transport, Tourism and Sport, Paschal Donohoe has said he was committed "to the tendering of 10% of bus routes".
Minister Donohoe said: "It must be noted that both Dublin Bus and Bus Éireann are open to compete for the routes being put out to tender.
"It is disappointing that the NBRU and SIPTU withdrew from discussions being facilitated by the Labour Relations Commission.
"Discussions had been aimed at addressing employee concerns about how the tendering of 10% of routes might impact them and were making significant progress."
Mr Donohoe said that during the talks it was made clear that employees that transferred to other operators would see their pension provisions maintained.
Bus Éireann said it was regrettable that the drivers were withdrawing from work during the busy bank holiday period, but said the company will try to minimise passenger disruption.
Dublin Bus strongly urged unions not to take action that could have a negative impact on customers.
The National Transport Authority also voiced disappointment at the union action, but sources said it would not be revising the schedule for the tender process.