Several 999 calls were also received as a result
The Coast Guard are reminding people that it is against the law to fire distress flares, unless it is an emergency.
It comes after a red parachute marine distress flare was fired in the Howth area of Dublin at around midnight on Saturday.
This caused several 999 calls and a report from a ship at sea.
While investigating with gardaí, a report was received of a flare being set off from a boat moored in the harbour.
Coast Guard personnel attended the scene, and confirmed no vessels were in distress.
In a statement, the Coast Guard says: "It is against the law to fire distress flares unless in an emergency as they signal that someone is immediate danger and needs assistance from the Coast Guard.
"Red distress flares are distinct in the sky, and will cause ships, and aircraft, and the public to alert the emergency services.
"False activation can tie up resources and delay the response to a genuine emergency call".