Monday's air traffic control failure in the UK was caused by "flight data" received by National Air Traffic Services (Nats), "with both primary and back-up systems responding by suspending automatic processing", its chief executive Martin Rolfe said.
The head of Nats also said that there is no evidence that the widespread disruption was the result of hacking.
"There are no indications that this was a cyber-attack", Mr Rolfe said.
He apologised for what her called "our technical failure", and insisted that "since yesterday afternoon all of our systems have been running normally", but acknowledged that "the knock-on effects at such a busy time of year" are continuing.
"Initial investigations into the problem show it relates to some of the flight data we received", Mr Rolfe added.
British aviation officials have opened an investigation into the issue.
'Very difficult day'
Ryanair boss Michael O'Leary has slammed British air traffic control provider for the "unacceptable" disruption caused over the past two days.
In a video message, the chief executive described yesterday as "a very difficult day" with 250 flights cancelled, while a further 70 were axed today.
The technical fault at the UK air traffic control yesterday disrupted hundreds of flights into and out of its airspace, with 115 flights to and from Dublin cancelled.
The issue started yesterday after a technical glitch meant that flight plans had to be inputted manually by controllers. This was identified and fixed by mid-afternoon, but the impact was felt long afterwards.
Mr O'Leary said: "We have been in contact with UK Nats. We still haven't had an explanation from them (about) what exactly caused this failure yesterday and where were their back-up systems.
"It's not acceptable that UK Nats simply allow their computer systems to be taken down and everybody's flights get cancelled."
Michael O'Leary said that Ryanair hopes to operate their schedule as normal tomorrow and said that up-to-date information would be on its website.
A number of its flights in and out of Dublin Airport, including flights to Liverpool, Naples and Riga, were cancelled this morning.
A total of 20 flights into and out of Dublin Airport have been cancelled today.
Dublin Airport operator daa said its team "worked through the night to help and support any passengers who were staying overnight in the terminals, providing blankets and refreshments, while several food and beverage outlets were kept open all night".
Media Relations Manager at daa Graeme McQueen said they are continuing to support airline partners and are working hard to ensure impacted passengers are able to resume their travel plans "as quickly and as safely as possible".
"Passengers intending to fly today are advised to check the status of their flight with their airline, before travelling to the airport," he said, adding that passengers needing to re-book flights are encouraged to do so on their airline's app or website.
"For any of our car park customers delayed coming back into Dublin Airport as a result of the ATC issues in the UK, we are waiving any additional parking fees they may have incurred."
Thousands of passengers were stranded in airports around Ireland and across Europe, including members of Team Ireland who had been at the World Athletics Championship in Budapest.
AirNav Ireland, which provides air traffic management services, began working with French air space controllers to help alleviate some of the delays by increasing the volume of aircraft using a section of air space that links Irish airspace directly to French airspace, therefore avoiding UK airspace
Many passengers managed to make new travel arrangements, while others cancelled holidays.
Airlines have apologised, saying the cancellations and delays were out of their control.
In a social media post last night, Aer Lingus said that it continues to face "unavoidable cancellations and delays".
"We are doing everything possible to minimise any further disruption to our customers and we apologise to all those impacted for the inconvenience caused, which is due to circumstances outside of our control," the airline said.
Cork Airport has also advised passengers to check the status of their flight with their airline before they travel to the airport.
At least 32 departures and 31 arrivals to and from Heathrow Airport have been cancelled, as the disruption continued.
British Airways, which operates the most flights to and from the west London airport, was the worst affected airline.
'Chaotic' scenes
Several passengers at East Midlands Airport booked overnight hotel stays at short notice due to the air traffic control disruption, in the hope they can depart today.
Michael and Chloe Kennedy, from Co Meath, were due to fly home yesterday after attending a festival but their flight cancellation meant they have had to take unpaid leave to cover the extra day and do not know whether they will be compensated.
Mr Kennedy said: "We queued for ages to leave our bags and just as we got to the front it was on our phones that the flight was cancelled.
"It was chaotic, (staff) were trying their best in there but a lot of people were not happy.
"Eventually we just left and booked a space in a hotel."
Ms Kennedy added: "Our flight was at 8pm and we were not told until 6.30pm, but we were lucky that we had not put our bag through security as some people were not allowed back through because of the risk, staff said.
"They had to return duty free items and things, it was all a bit mad."
Ms Kennedy said there was very little communication from the airlines or airport staff about the cause of the delays.
She said: "Nobody knew anything. There should have been someone shouting what the problem was, but we still had to queue and queue and queue before we got any answers.
"It was unnecessary and chaotic."
British transport secretary Mark Harper said it was the worst incident of its kind in "nearly a decade" and announced an "independent review" will be carried out.
"The Civil Aviation Authority will be putting together a report in the coming days, which obviously I will take a look at to see whether there are lessons to learn for the future, to see whether we can reduce the impact of this again," He said on GB News.
