Public transport users face the threat of serious disruption this summer following the decision of the National Bus and Rail Union (NBRU) to proceed with a strike ballot.
The other main CIÉ union, SIPTU, which has yet to take a position on the NBRU strike threat, meets today to review developments.
Industrial action by either union would shut down services, as members of each traditionally support actions taken by the other.
At a meeting of its executive committee on Saturday, the NBRU decided to ballot members on industrial action over the next three weeks.
It would be at least a month, however, before services were disrupted as a week's strike notice would be served on completion of the ballot.
Even at that stage a total shutdown of services would be unlikely.
"It is envisaged that a series of roll-over strikes will take place, leading ultimately to an all-out strike," said the union's general secretary, Mr Liam Tobin.
Unions in CIÉ are opposed to plans by the Minister for Transport, Mr Brennan, to hand over up to 25 per cent of the Dublin bus market to private operators.
Since March they have been engaged in talks with his Department, chaired by Mr Kevin Foley, of the Labour Relations Commission.
The NBRU strike threat followed a decision by the Department to award licences to a private operator, Morton's Coaches, for two new services to the CityWest business campus.
Mr Tobin, speaking after Saturday's meeting, said Mr Brennan was "effectively privatising the network route by route while we talk".
The Department, he said, had been unable to provide a definitive answer as to when it could respond to the unions' concerns over the future of Dublin Bus and the other CIÉ companies.
"They have told us that they intend to deal with Aer Rianta first. In the circumstances our own talks could go on indefinitely. Meanwhile they have told us they will continue to issue licences to private operators."
