Strike action at Bus Eireann and Dublin Bus next month could cost the companies up to 5 million euro.
Bus Eireann says it's a huge loss and wants more talks to try and hammer out a solution.
The NBRU and Siptu yesterday announced a combined total of 7 days industrial action in May, over proposals to privatise some routes on the network.
SIPTU says: "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," it adds.
The union also says it has met with the Department of Transport - which it described as "useful" - but adds that until they meet with both bus companies "it is impossible to make an assessment on the prospects of making progress in allying our members' concerns."
SIPTU utilities and construction division organiser, Owen Reidy, told Newstalk Lunchtime the bus companies will not meet them - so they feel they are left with no choice but to strike.
General Secretary of the National Bus and Railworkers Union Dermot O'Leary wants action from the Minister for Transport, after Paschal Donohoe yesterday called for the strike to be called off:
