A plane arrives at Belfast International Airport
Flights to and from Northern Ireland and the Irish Republic have resumed after airspace was closed due to volcanic ash drifting from Iceland.
Flights were grounded from 0700 BST over predictions ash would exceed acceptable levels for jet engines.
About 6,000 passengers at Belfast International Airport faced disruptions to their flights.
But the Civil Aviation Authority and the Irish Aviation Authority said services could resume at 1300 BST.
Flights from the UK and continental Europe flying across the airspace were not affected.
The Civil Aviation Authority said it had been closely monitoring the situation and was taking advice from the Met Office.
A spokesman for Belfast International Airport said the backlog was expected to clear in good time, with passengers being placed on other flights.
Some charter flights which were postponed on Tuesday morning are now due to leave at about 1800 BST.
The spokesman said there could be further disruption to flights later as concerns have been raised about Scottish airspace, but no stoppage has yet been put in place.
Speaking on Tuesday morning, Belfast City Airport operations director Mark Beattie said it was "a hugely frustrating time for everyone".
The IAA said the decision to close its airspace was based on information from the Volcanic Ash Advice Centre (VAAC).
The body said the move to ground aircraft was based on the safety risks to crews and passengers as a result of the drift south of the volcanic ash cloud caused by the north-easterly winds.
Flights over Europe were banned for six days last month because of fears of the effect of volcanic ash on plane engines.
The decision to lift the restrictions followed safety tests that showed the engines could cope in areas of low density ash.
The fresh disruption came as European Union transport ministers met in Brussels to discuss ways to improve air traffic management in the wake of the events of last month.