SIPTU is to enter new talks on the future of CIÉ, but has also threatened public transport disruption next month.
The union's Dublin Bus, Iarnród Éireann and Bus Éireann strike committees yesterday decided "with great scepticism" to take part in negotiations with the Department of Transport.
Unless the talks proved "meaningful", however, it would stage industrial action on March 18th, it said.
Mr Michael Halpenny, national industrial secretary of the union, said the type of action had not yet been decided.
"Basically we're saying that if this turns out to be another meandering series of talks going nowhere, then industrial action will take place.
"Equally, if there is a meaningful talks process and meaningful engagement with the issues, obviously we will suspend the action."
The other main CIÉ union, the NBRU, has already issued a positive response to a letter from the Department of Transport this week inviting the unions to talks.
Unions are opposed to plans by the Minister for Transport, Mr Brennan, to dismantle CIÉ and privatise some routes currently operated by Dublin Bus and Bus Éireann.
In the letter to the unions, the Department's secretary general, Ms Julie O'Neill, said the Minister was prepared to consider "additional options".
Talks are expected to get under way quickly following the appointment of an agreed independent chairman. Mr Brennan said he wants them to be conducted "within a tight time- frame".
Yesterday's meeting of the SIPTU strike committees lasted for more than four hours.
Mr Halpenny said the thinking of members had been conditioned by the unsatisfactory nature of previous talks with the Department. Strong concerns had also been expressed at the meeting about the future of the CIÉ companies and the jobs of those who worked for them, he said.
It is understood that the union's threat of industrial action will not prevent the talks from going ahead. Mr Halpenny said that was a matter for the Minister. "There is no point in glossing over the depth of feeling that is out there."
Meanwhile, about 2,000 passengers on trains to and from Westport, Co Mayo, had their journeys disrupted again yesterday by a second one-day strike by drivers. Six drivers, four of them members of SIPTU and two from the ATGWU, are engaged in a dispute with Iarnród Éireann over rosters.
Bus services were provided for passengers affected by the unofficial dispute, which forced the cancellation of some services between Westport and Roscommon, and between Westport and Athlone.
A one-day strike was also staged on Friday of last week, and similar actions are likely.
