Flights between Ireland and Scotland were cancelled on Tuesday as the return of the Icelandic ash cloud threatened travel chaos.
Both Ryanair and Aer Lingus halted services to Glasgow, Edinburgh and Aberdeen.
Ryanair said it strongly objected to the disruption under orders from the Irish Aviation Authority (IAA).
The IAA said on Monday night that it did not expect any disruption at Irish airports during most of Tuesday.
But it later ordered flights to Scotland to be pulled as forecasters predicted that the volcanic plume, billowing from Iceland's Grimsvotn volcano, would hit Scotland and Northern Ireland during the morning.
Ryanair insisted there was no basis for the flight cancellations. It was due to meet the IAA to have the restrictions removed as a matter of urgency, according to its website.
"Ryanair believe that there is no safety risk to aircraft on fights operating to and from Scotland and, together with other airlines, will be complaining to the Transport Minister and regulatory authorities about these latest and unnecessary cancellations," a company spokesman said.
Both Ryanair and Aer Lingus advised intending passengers to contact their airline or check the company websites ahead of travelling to airports.
The IAA said that, based on the latest information from the Volcanic Ash Advisory Centre (VAAC) in London, there would be no shutdown of Irish airspace.
"All Irish airports remain open but some services may be affected by restrictions imposed by other European service providers," said a spokeswoman.