The Air Corps helicopter that lost its door after an emergency medical airlift in May was at the centre of the same type of incident 11 years ago.
A preliminary report on the incident confirmed that the Agusta Westland helicopter was the same chopper that lost a different door over Co Kerry in 2009, while transporting then tourism minister Martin Cullen to an event.
The report also details how the 14-year-old helicopter had originally been on its way to collect a patient for a transfer to Tallaght Hospital in May of this year. While en route, the National Ambulance Service asked the chopper to fly instead to the Phoenix Park, so the patient could be moved to the Mater Hospital.
The report said: "The heli then routed back to Casement [Air Corps headquarters] for fuel, prior to returning to Athlone for the evening.
"While routing to Casement, the right door of the heli[copter] became detached over Clondalkin and fell to the ground in Moyle Park College."
The report confirmed it was the same aircraft that had lost a door in March 2009 while flying over Killarney. On that occasion, it was the front left door that detached.
Records show the Air Accident Investigation Unit, who investigate serious air incidents, had been contacted.
"[They] declined to be involved in the investigation given the nature of the incident," the report said. "They did offer their assistance if needed, for the download and analysis of the flight data."
The report - prepared for then Minister of State Paul Kehoe by Brigadier General Rory O'Connor - said there were no other state authorities investigating the incident.
On the flight prior to the latest incident, there had been five people on board - three crew, an advanced paramedic, and the patient who was being transferred.
However, by the time the door fell off, there were only three on board - two pilots and a crewman. The paramedic had accompanied the patient to the Mater Hospital.
The report stated: "While routing over the Clondalkin area, the right-hand side door became detached from the aircraft and fell to the ground, striking the roof of an unused building before falling to the ground."
No one was hit by any falling debris, the report said.
By the time the chopper was back at Casement aerodrome, a member of the public had called the Defence Forces.
The report said: "The aircraft was secured in its hangar so that flight data, voice recorder, etc. could be downloaded for examination."
Personnel from the Air Corps Flight Safety Section went to the incident site to carry out their investigation and remove any debris.
Defence Forces spokesman Captain Risteard Byrne said: "[We] can confirm the same aircraft, tail number 278, was involved in an incident in 2009 and an incident in May 2020."
Asked about the investigation's status, he said: "The Defence Forces cannot comment on the status of the report as it is currently ongoing."