Irish airports were due to reopen Tuesday after being shut down for six hours due to the return of volcanic ash from an Icelandic volcano which brought chaos to thousands of travellers last month.
The Irish Aviation Authority (IAA) said seven airports including Dublin and Shannon would reopen for normal operations from 1:00 pm (1200 GMT) after they were shut from 0600 GMT as a precautionary measure.
But its chief executive Eamon Brennan warned that Ireland "remains at risk" of further disruption in the days and weeks to come.
"The reason we have changed our advice is because the ash cloud has moved towards the south, down towards the Bay of Biscay," he told RTE state radio.
"At the moment the volcano is more or less dormant but should it re-erupt again we'd be faced with this problem."
There were also closures in Northern Ireland and over the Outer Hebrides, a group of islands off the western coast of Scotland, imposed by Britain's Civil Aviation Authority (CAA).
These remain in place but the CAA was expected to make an announcement soon after it receives fresh information from weather forecasters.
The new alerts did not disrupt aircraft flying over Ireland from Britain or Europe, or southern British airports including Heathrow, Europe's busiest air hub, authorities in the two countries said.
Airspace across Europe was closed down for up to a week last month after the eruption of Iceland's Eyjafjallajokull, but was re-opened after emergency talks between European governments, airlines and regulators.
The new closure triggered the cancellation of hundreds of flights in and out of Ireland and Northern Ireland, bringing fresh chaos to thousands of people.
Heathrow said that around 20 flights to and from the west London airport had been cancelled this morning.
"We are asking passengers to check with their airlines before coming to the airport," said a Heathrow spokeswoman.
Budget Irish carrier Ryanair said it had cancelled all flights into and out of Ireland between 0500 GMT and 1300 GMT Tuesday.
"The first wave is clearly one of the busiest parts of the day so it will have a fairly significant effect on the operation tomorrow," said airline spokesman Stephen McNamara.
Irish flag carrier Aer Lingus said it had cancelled all British and European flights scheduled to depart and arrive into Dublin and other airports in the country until 1200 GMT Tuesday.
The international airline industry body, IATA, said last month's shutdown cost carriers some 1.7 billion dollars (1.3 billion euros) and called on governments to pick up at least part of the cost.
Eurocontrol, the continent's air traffic control coordinator, said more than 100,000 flights to, from and within Europe had been cancelled between April 15 and 21, preventing an estimated 10 million passengers from travelling.