Unions in Dublin Bus and the Minister for Transport, Mr Brennan, remain on a collision course over his plans to open the capital's bus market to competition.
Talks between the two sides have ended without agreement, and one of the unions concerned, the National Bus and Rail Union (NBRU), is preparing to ballot its members on industrial action.
Its decision to proceed with a ballot followed an interview with Mr Brennan in The Irish Times on Friday, in which he predicted a possible "summer of discontent" but said he was pressing ahead with his plans for the Dublin bus market.
He aims to have 25 per cent of this market opened to competition by next January.
The NBRU said it was "astounded" at the Minister's comments and had been left with little choice but to act.
"We have been told in talks with senior officials that the Minister is not only considering handing over Dublin Bus routes to private operators, but possibly Dublin Bus vehicles and garages as well.
"This even though he admits Dublin Bus and its staff have done a good job in appalling conditions," said the union's general secretary, Mr Liam Tobin.
"His decision flies in the face of experience in other European cities, which shows that public bus schedules experience a rapid deterioration in the quality of service and massive fare increases when handed over to private operators."
Mr Tobin's union, as well as SIPTU and the Irish Congress of Trade Unions general secretary, Mr David Begg, have held meetings with officials in Mr Brennan's Department on his plans in recent weeks.
