British and Irish airspace reopened Tuesday after the return of a volcanic ash cloud caused more travel disruption.
The restrictions prevented thousands of travelers from flying and passengers have been warned to check the status of their flights with airlines as there could still be delays.
Budget carrier Ryanair is putting on extra flights on Wednesday to help those caught up in the latest disruption.
The Irish Aviation Authority (IAA) expected operations to quickly return to normal at Irish airports but said there could be more misery this week and a "summer of uncertainty" ahead.
One of the first people to fly into Northern Ireland once the restrictions were lifted was Conservative leader David Cameron, who was there for the general election campaign.
The flight ban now only remains in a very small no-fly zone identified by the Civil Aviation Authority (CAA) in the northwest corner of U.K. airspace.
"This no-fly zone is not expected to have any impact on U.K. operations," air traffic controllers Nats said.
Flights from continental Europe and transatlantic services passing over Ireland and Northern Ireland were not affected by the plume of ash from Iceland's volcano Eyjafjallajokull, which caused travel chaos for almost a week in April.