Flights have been cancelled at all Northern Ireland airports
The no-fly zone that has been affecting flights at Northern Ireland's airports has been lifted after fresh information from the Met Office.
Flights were able to operate from all three of Northern Ireland's airport from 1300 BST, but delays and cancellations are still possible.
Passengers due to travel are still advised to check with their airline.
The easing of the no-fly zone means Heathrow and Gatwick airports in London have also reopened.
From 1300-1900 BST the no-fly zone remains in place only in the Orkney and Shetland Islands.
The UK no-fly zones are set out by the Civil Aviation Authority using Met Office data.
Flights in and out of Dublin, in the Irish Republic, resumed from noon.
A City of Derry Airport spokesperson said scheduled services to Birmingham with Ryanair and the Dublin service with Aer Arann on Monday evening were expected to operate normally.
Prof Brian Golding, head of forecasting research at the Met Office, said the cloud stemmed from an eruption on Thursday.
"The volcano has now dropped back in height. It did that yesterday, so the ash coming towards us for the future isn't quite so deep as it was on Thursday.
"It isn't going to turn into a huge area and it's being blown eastwards, between south-east and east. "
The air traffic authority Nats' Jonathan Astill said: "Unfortunately, yet again, a mixture of volcanic activity and weather systems have conspired to bring a cloud of ash down towards the UK."
Peter Gibbs explains how the ash cloud is moving in the UK's airspace
In April, airspace across Europe was shut down for five days over concerns ash from the Eyjafjallajokull volcano could turn to molten glass in high temperatures, crippling plane engines.
Scientists and engineers have since revised the safe-to-fly threshold, but clouds of volcanic ash have continued to drift over Europe, causing airport closures, flight delays and cancellations.
In the past week, several airports in southern Europe have been forced to close and flights have been re-routed.