Members of the National Bus and Rail Union have voted by a huge majority to take industrial action over plans by the Minister for Transport, Mr Brennan, to privatise some of the Dublin bus market.
Similar moves are expected by other unions, but there is no immediate threat of transport disruption.
In a ballot concluded yesterday, 94 per cent of NBRU members in CIÉ, mostly bus and train drivers, voted in favour of industrial action.
The union has about 3,600 members in CIÉ, about half of whom work for Dublin Bus.
"The result of the ballot showed the depth of feeling and anger of our members at 'the lengths to which the Minister will go to leave our public transport system at the mercy of a globalised market'," said the union's general secretary, Mr Liam Tobin.
"We will not see services taken over by faceless multi- national businessmen in Singapore and London."
Industrial action is unlikely to take place, however, until at least after the NBRU executive meets on May 20th.
Mr Brennan plans to dismantle CIÉ and open 25 per cent of the Dublin bus market to competition by next January.
In an interview with The Irish Times last month, he said there might "be a summer of discontent", but he was "pressing ahead" with his plans as set out in the Programme for Government.
Dublin Bus, as well as the travelling public, would benefit from the introduction of competition into the market, he said.
Mr Tobin claimed yesterday that Mr Brennan had been pursuing privatisation since he first became Minister for Transport 14 years ago.