Industrial action by CIÉ workers planned for next Tuesday was called off yesterday following talks between unions and the Department of Transport.
A series of one-day stoppages planned for the autumn have also been deferred pending the outcome of "substantive discussions" next month.
The actions had been organised in response to plans by the Minister for Transport, Mr Brennan, to dismantle CIÉ and introduce competition to some of the routes operated by Dublin Bus.
Yesterday's surprise breakthrough came during talks between union leaders and officials of the Department of Transport, including its secretary-general, Ms Julie O'Neill.
Mr Brennan was not present but was briefed on the outcome, a Department spokeswoman said.
In a joint statement, the two sides said they had agreed to a phased series of discussions during September and October.
These would be designed "to address the concerns of the trade unions in a meaningful way in the context of the objective of securing value for money from public transport services through increased efficiency and better value for customers.
"Both parties agreed to use this period to engage in substantive discussions on the complex issues involved.
"In light of the above, the unions have decided to defer the planned industrial action pending the outcome of the process," the statement concluded.
Mr Liam Tobin, general secretary of the National Bus and Rail Union, said the attitude of the Department officials had been positive.
"They appeared to be taking on board in a serious fashion the concerns of the trade unions and our members for the first time," he said.
Ms O'Neill had promised to do everything possible to ensure the talks process would be followed through to the end, he added.
The talks are to begin on September 10th and may continue until after October if progress is made.
While these talks continue, the unions will refrain from engaging in industrial action.
Unions had threatened to follow next week's planned half-day stoppage with a series of one-day strikes, culminating in a two-day strike in December.
Their campaign had begun last month with a "no-fares day", which resulted in free public transport but a loss of €1.2 million in potential revenue for Dublin Bus, Iarnród Éireann and Bus Éireann.
Unions are particularly opposed to Mr Brennan's plan to open 25 per cent of the existing Dublin bus market to competition by private operators by early next year.