Dublin Airport was forced to suspend all flights due to what it described as a "confirmed sighting of a drone" over its airfield.
A pilot on the airfield spotted the suspected drone at around 11.30am on Thursday and reported it to the
Irish Aviation Authority, who made the decision to close the runway.
In a statement released on Twitter, the airport said: "For safety reasons we are temporarily suspending flight operations Dublin Airport due the confirmed sighting of a drone over the airfield.
"Passengers should contact their airline's website for flight updates. We will post updates here when they become available."
But just 15 minutes later, the airport announced "flight operations" had resumed.
The statement added: "We apologise for any inconvenience. The safety and security of passengers is always our key priority."
Airport police were seen questioning motorists at a site near the airport which is popular with plane spotters and the Garda confirmed in a short statement the incident was under investigation.
In a statement on Thursday afternoon, Dublin Airport said: "Flight operations at Dublin Airport were suspended for safety reasons for a short period this morning following a confirmed drone sighting on the airfield.
"A pilot reported a drone sighting to the Irish Aviation Authority (IAA), which operates air traffic control at Dublin Airport, at about 11.30am.
"Very shortly afterwards, and having contacted other aircraft in the vicinity, the IAA suspended flight operations at Dublin Airport in line with agreed protocols for confirmed drone sightings.
"A 30-minute suspension of flights was implemented by air traffic control at that point, which is the agreed procedure in such cases. As there were no further drone sighting within the 30-minute suspension period, Dublin Airport resumed flight operations shortly after noon.
"The Gardai and other State agencies were informed of the incident immediately. It is illegal to fly drones within five kilometres of an airport in the State and all drones that weigh more than 1kg must be registered with the IAA.
"The safety and security of airport users is daa’s key priority at all times and staff at Dublin Airport, the IAA, and other agencies continue to remain vigilant in relation to drone activity in the vicinity of the airport.
"Three flights were diverted due the suspension of flight operations and there will also be some knock-on delays to flights today."
In January, Ireland's Transport Minister Shane Ross convened a meeting of experts to discuss the threat posed by drones.
He said while the prospect of an attack was "low", authorities needed to be vigilant.
The National Civil Aviation Threat and Risk Group meeting involved the Garda, Defence Forces, officials from the departments of transport, justice and foreign affairs, the Irish Aviation Authority, and representatives from Ireland's airports and airlines.
After Thursday's incident, Ireland's main opposition party Fianna Fail called for more government action.
Transport spokesman Robert Troy TD said: "In the first instance I am glad that the disruption to flights in Dublin Airport was kept to a minimum and that all flights have resumed, however let there be no doubt about it, it could have been much more chaotic.
"Minister Ross's previous comments that the likelihood of a drone attack in Ireland was 'low' clearly shows his lack of understanding of the seriousness of the issue.
"Following the drone sightings at Gatwick Airport, he was at pains to stress that the regulatory provisions in place in Ireland, which control and restrict the use of drones, were stronger than those of the UK.
"His dismissive attitude and the failure of the Government to progress the Fianna Fail Bill to regulate drone usage which was brought forward by my party colleague James Lawless, will lead to further disruptions such as was seen in Dublin Airport today.
"Having our air space compromised by the threat of drones is not something they can continue to ignore."
It comes just two months after reported drone sightings caused havoc at Gatwick airport.
The West Sussex airport was on lockdown for 36 hours in December, disrupting more than 140,000 passengers.
Passengers eventually got on the move again on December 21, but three days later there was more disruption when another drone sighting saw flights grounded at Heathrow, an incident for which a man has been charged.