Passengers at Dublin Bus and Bus Éireann face serious disruption as unions at the two companies escalated their plans for industrial action over plans to put 10% of routes out to private tender.
SIPTU and the National Bus and Rail Union are now planning to hold two 48-hour stoppages on 1 and 2 May, as well as 15 and 16 May.
They are also planning a three-day strike on 29, 30 and 31 May.
Unions at the two State bus companies fear that if 10% of bus routes are opened up to the private sector, it could trigger a deterioration in the pay and conditions of members.
The NBRU had already announced a two-hour stoppage on 1 May - the Friday of the bank holiday weekend.
However, today SIPTU announced it would hold two 48-hour strikes on 1 and 2 of May, as well as 15 and 16 May.
By this afternoon, it emerged that there will also be a three-day strike on the last three days of the month.
Minister for Transport Paschal Donohoe warned the action would hurt the economy, inconvenience commuters and cause widespread disruption.
He restated his support of the two companies but reiterated his commitment to what he called competitive tension in the marketplace.
Meanwhile in the courts, the NBRU was granted leave to seek judicial review of the legislative basis for the National Transport Authority's plans to put routes out to tender.
Bus Éireann has said it strongly urges the trade unions to reconsider and avoid any industrial action that will adversely impact its customers.
In a statement, it said: "We are also particularly mindful of students starting exams around this time, and many of those who will have travel plans for a busy bank holiday weekend.
"Bus Éireann has responded to every request to participate in the Labour Relations Commission (LRC) discussions regarding the decision by the NTA (National Transport Authority) to tender 10% of bus routes.
"We agreed to SIPTU's recent request to meet regarding their issues, and stated that we would do so under the auspices of the LRC, and we still remain available to meet any of the parties involved in the ongoing process."
SIPTU Utilities and Construction Division organiser Owen Reidy said: "As a result of talks at the Labour Relations Commission (LRC) collapsing, the SIPTU Dublin Bus and Bus Éireann joint sub-committee met on 13th April.
"At this meeting it was decided to postpone any decision on industrial action for ten days to facilitate meetings with both bus companies and the Department of Transport.
"In correspondence received from Bus Éireann on 16th April and from Dublin Bus on the 17th April, the companies refused to agree to a meeting and merely recommended that the LRC process should be reconstituted.
"It is outrageous that having taken a moderate approach to prevent industrial action by seeking direct talks with both bus companies, SIPTU has been met with such intransigence."
Speaking on RTÉ's Six-One News Mr Reidy said the union needs clarity and certainty and that if government continues to insist on privatisation, the union wants to address the consequences of privatisation.
He said the union's view is that the National Transport Authority should press pause on the tendering process until questions can be answered.
NBRU General Secretary Dermot O'Leary said: "Whilst we have previously announced our intention to keep the disruption to the public at a minimum on the 1st May, it now appears that initial positive statements from the Minister have not been followed up by actions to get the parties responsible for this anti-bus worker, anti-travelling public policy to recognise and address the serious issues at play.
"This, along with the welcome show of solidarity by our trade union colleagues has led to the NBRU outlining a significant programme of industrial action through the coming months."
He added: "The proposed action will as a matter of course dovetail with our trade union colleagues with a further 48-hour stoppage on the 15th & 16th May followed by a three-day work stoppage on the 29th May, 30 & 31st May, the resolution to a protracted dispute along with the prospect of a summer of discontent rests with the Government."