Passengers at NI's airports have been experiencing delays due to increased security after a suspected plot to blow up planes over the UK was disrupted.
Travellers will not be able to take hand luggage with them onto planes.
Belfast International Airport is advising passengers to check with their airline before leaving home.
Flights at George Best Belfast City Airport are getting back to normal. City of Derry says check-in on flights will close one hour before departure.
Passengers at Dublin Airport are being allowed to take hand luggage on board, but those travelling from UK airports into Dublin have been warned to expect extra delays.
Security at all airports in the UK was stepped up after police made arrests following an anti-terrorist operation lasting several months.
CABIN LUGGAGE ALLOWED
Pocket-sized wallets and purses, plus contents
Passports and travel tickets
Prescription medicines, not in liquid form unless can be verified as authentic
Essential medical items such as diabetic kit
Glasses and sunglasses, no cases
Contact lens holders, no bottles of solution
Baby food and milk for those with infants - contents of each bottle must be tasted by accompanying passenger
Essential sanitary items for infants
Female sanitary items, unboxed
Tissues, unboxed, or handkerchiefs
Keys, but no electric key fobs
MI5 has raised the UK threat level to critical - the highest possible.
According to MI5's website, critical threat level means "an attack is expected imminently and indicates an extremely high level of threat to the UK".
Belfast International Airport security director Robin Long said delays would be inevitable.
"It will help all of us if passengers are prepared," he said.
"Unusually, we are searching absolutely everybody, so that is having a massive impact."
John Devine, manager of City of Derry Airport, appealed for anyone flying from the airport to turn up as well prepared as possible.
"We are experiencing delays to out-bound and, more importantly, inbound flights - we haven't received any in-bound flights from any UK airports.
"But I have to say passengers have caught on very quickly to this.
"They are approaching the check-in desks well prepared and are repacking all of their hand luggage into their hold luggage, and I have to say it is running fairly smoothly at this moment."
Ryan Byrne was due to fly from Londonderry to Liverpool for a family wedding, but his flight was cancelled.
"We were due to fly out at 2.15pm to Liverpool to go to my cousin's wedding tomorrow, the flight has been cancelled.
"Everything is up in smoke and my cousin is distraught. We are going to have to try and sort something else out."
The Department for Transport said passengers will not be allowed to take any hand luggage on to any flights in the UK.
Only the barest essentials - including passports and wallets - will be allowed to be carried on board in transparent plastic bags, the department said.
Meanwhile, Heathrow Airport has been closed to all incoming flights that are not already in the air, while several outbound services have been cancelled.