Operations at Dublin Airport were suspended for 30 minutes after a pilot reported a drone flying above the airfield.
Planes were grounded and incoming flights diverted after the incident at around 11.30am on Thursday.
The Irish Aviation Authority (IAA) said a 30-minute suspension was ordered in line with official protocols for drone sightings.
No more drone activity was reported in the period and operations resumed.
After the incident, airport police and gardai, including members of the armed response unit, were seen conducting inquiries close to the perimeter of the airfield.
Officers spoke to a number of people and searched vehicles in an area popular with plane spotters on Old Airport Road.
Three incoming flights were diverted due to the scare, two to Belfast and one to Shannon.
Siobhan O'Donnell, spokeswoman for Dublin Airport, said the drone had not been recovered and she expressed concern it could return.
She added: "A pilot on the airfield spotted the drone and contacted the Irish Aviation Authority so they took the decision then to suspend flight operations.
"It's a very serious situation to spot a drone in the airfield so the decision to suspend flights was not taken lightly.
"Drones can do huge damage to an aircraft. The drone was out in the airfield area where flights are landing and taking off."
An IAA spokesman said: "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 (Dublin Airport Authority) 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."
Dublin Airport apologised for the inconvenience to passengers, tweeting: "The safety and security of passengers is always our key priority."
Aer Lingus said passengers using Dublin Airport may experience delays.
Before Christmas, drone sightings caused travel chaos at Gatwick Airport, near London. Aircraft were grounded over three days, with 1,000 flights cancelled and 140,000 passengers affected.
In January, Ireland's Transport Minister Shane Ross convened a meeting of experts to discuss the threat posed by drones.
At the time he said the prospect of an attack was "low" but 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.
Ireland's deputy premier Simon Coveney reacted to the incident in the Dail parliament on Thursday afternoon.
"There are some very sophisticated airports in some very well resourced countries that haven't been able to deal with this issue easily, in the US and the UK most recently," he said.
"I don't think we should pretend that Ireland should be immune from challenges that are very difficult to deal with from a technical perspective.
"But we of course will learn from the lessons of others, particularly our closest neighbour, and ensure that we put mechanisms in place that are as effective as possible."
Ireland's main opposition party Fianna Fail called for government action and more robust regulations.
Transport spokesman Robert Troy TD said: "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."