Aer Lingus and Aer Rianta will suffer a severe loss of business following the carnage in the US when hijacked planes smashed into the World Trade Centre and the Pentagon.
With the global aviation industry already in decline this year, both companies said the situation was "very serious".
But Ryanair declined to comment when asked what impact the US attacks would have on its business. The airline's stock closed more than 14 per cent weaker at €8.33 (£6.56) on the Dublin exchange yesterday, a day after it lost 7 per cent.
Citing steep falls in other aviation stocks, Dublin-based analysts said airlines worldwide faced seismic downturns as Americans and other passengers shirk flying after the devastation wrought in New York, Washington and Pittsburgh.
Sources at Aer Lingus, which lost €38 million in the first half of the year with more to follow, acknowledged the situation was grim.
The State-owned airline was bailed out by the Government when the Gulf War prompted its last crisis, but that option has been ruled out by the European Commission.
A recovery plan being developed centred on the need to engineer a recovery before next summer and that would now be changed, sources said.
The looming downswing was likely to be much more critical than during the Gulf War, which saw the industry lose more money in three years than the profits made in the previous 70.
A spokesman said: "It's going to accelerate the downturn in the business. The only real question is how serious and how long this is going to last."
An Aer Rianta spokesman said: "It's going to have a very, very severe impact ... It was frightening. It's a great human tragedy. From a business point of view, it's definitely very bad."
The managing director of a London consulting firm, Aviation Economics, said the industry faced a "litany of disasters". Mr Keith McMullan said: "I suppose nobody knows what's going to happen, but I guess there'll be a precipitous fall in traffic, major cash flow difficulties, and rising insurance and fuel costs."
This would prompt questions about the very viability of large numbers of US and European airlines, already suffering due to easing economic growth in the US.
A senior figure in a Dublin corporate finance house said: "This can be nothing but seriously detrimental to the global aviation industry. We know Aer Lingus is suffering the consequences of the downturn. This is the worst possible thing that could happen from an Aer Lingus perspective."
