Serious disruption to services at the State's three main airports is expected from lunchtime on Monday in response to the planned break-up of Aer Rianta announced yesterday.
Unions reacted with extreme anger to the move, with one leading official, Mr Jack O'Connor, accusing Minister for Transport, Mr Brennan, of "institutionalised vandalism".
Mr O'Connor, vice-president of SIPTU, also claimed Mr Brennan's plans were reckless and defied commercial logic.
Union leaders are to meet at the Irish Congress of Trade Unions headquarters in Dublin on Monday morning to plan official opposition to the break-up.
Events are likely to be overtaken, however, by general meetings of workers at the three airports - Dublin, Shannon and Cork - starting at 2 p.m.
Organised by SIPTU, the "information meetings" are expected to be attended by members of all unions at the airports and have the potential to cause at least a temporary shutdown of services.
Mr Dermot O'Loughlin, secretary of SIPTU's civil aviation branch, said there would be "no compulsion" on workers to attend the meetings.
"But, in view of the seriousness of the proposals, it can be assumed that there will be massive turnouts," he said.
It is not known how long the meetings will last but an early return to work by staff is considered unlikely.
Union leaders' anger at yesterday's announcement is compounded by the fact that many felt reassured by comments made by the Taoiseach, Mr Ahern, to last week's ICTU conference in Tralee.
Mr Ahern said he had no desire for conflict with the unions and he believed change could be implemented in a "spirit of partnership".
He had earlier, in a letter to ICTU general secretary Mr David Begg last February, said the Government was committed to "active engagement" with the social partners on the future of the commercial semi-state sector.
Mr O'Connor said undertakings given by the Taoiseach would have provided a means by which crucially important economic decisions could have been evaluated in a transparent way.
Instead, Mr Brennan and the Government had chosen to ignore commitments given under Sustaining Progress and "make hasty decisions instead".
"The decision will have the most far-reaching and negative consequences for jobs and standards of employment in all three locations, and will dramatically reduce the value of this profitable national asset," he said.
The ICTU expressed "strong reservations" about the "potential job losses" in Cork and Shannon, and questioned the viability of these airports without the support of Aer Rianta.
Mr Begg said that breaking up Aer Rianta would "weaken" the company and reduce its capacity to operate internationally.
Mr Arthur Hall of the Technical, Electrical and Engineering Union (TEEU) described the move as "economic madness".
"It is going to cost the taxpayer huge sums of money. The debts transferred from Shannon and Cork to Dublin will damage the latter and the other airports have no long-term future without Dublin."
