Shannon Air Traffic Control received a call from a British Airways pilot flying from Montreal to Heathrow over County Kerry at 6.47am on 9 November.
She said the object came up alongside the plane before vanishing at "a very high speed".
"Alongside you?" the Shannon Air Traffic Controller replied.
"It came up on our left-hand-side and then rapidly veered to the north," the BA pilot said.
She said it was "a bright light and then it disappeared at a very high-speed", adding they were "wondering" what it could be - adding that that it did not seem to be on a collision course.
Controllers on the ground said there was "nothing showing on either primary or secondary (radar)".
Two other planes can then be heard reporting the sighting to Shannon Air Traffic Control.
A Virgin Airlines pilot suggested that the it could be an object re-entering the earth's atmosphere after reporting seeing "multiple objects following the same sort of trajectory... very bright from where we were."
Another pilot said the speed of the UFO was "astronomical, it was like Mach 2", or twice the speed of sound.
Despite the term UFO being widely used for extra-terrestrial spacecraft, most UFOs are later identified as conventional objects or phenomena.
The Irish Aviation Authority said they were investigating the incident.