GSOC said an independent investigation was “necessary to ensure public confidence” in policing oversight at a time of increased armed gardai in Dublin in particular.
The Policing Authority and the Minister for Justice have also expressed serious concern and are to be briefed on an internal Garda investigation already underway.
The authority has also sought a “comprehensive” briefing on garda procedures in the carrying of firearms generally and is set to discuss the matter at their next meeting.
The incident occurred as three-to-four vehicles attached to the Armed Support Unit departed from Dublin Garda HQ at Harcourt Square, in the south inner city, and drove down Harcourt Street at 5.30pm on Tuesday.
A woman who was driving behind the convoy spotted a satchel, which contained the Heckler & Koch MP7 machine gun and magazines, on the street, picked it up and put it into her car.
Seeing it was a garda satchel she drove across the city centre, over the River Liffey, to Store Street Garda Station and handed it in – some 20 minutes later.
Chief Superintendent Lorraine Wheatley has been tasked with examining the circumstances of the incident and will try and determine if the incident was the result of a technical problem with the vehicle, human error or a mixture of both, or some other factor.
There were conflicting and confusing reports over the day as to what happened. The examination team will now talk to garda members involved, the woman, gather CCTV from relevant areas and conduct an examination on the vehicle concerned.
There were initial reports the firearm was actually loaded but senior sources said it was not, but contained in a satchel along with ammunition.
In the new AudiQ7 and BMWs purchased for the ASUs, firearms are stored in fitted compartments in sliding drawers in the boot, but if members are in unmarked vehicles the firearms could be held in satchels, sources said.
Reports differ as to where the satchel was at the time - that it fell out of a boot that was opened, possibly because the boot failed to close, or there was a technical fault with it, or it opened after going over a ramp.
There were later reports the satchel was in the car and fell out of a door. Investigators will also look at the possibility the satchel may have been elsewhere when the car drove off.
There was also some confusion when and how the ASU members discovered the satchel was missing. Harcourt Street is a one-way street, with Luas lines running on the other side of the road, making it difficult to turn back.
In a statement, GSOC said they had become aware of the matter through media reports and, because they had not received any referral from Gardai or from the public, had decided to launch a public interest investigation.
It said this can be done where it appears a garda “may have” committed an offence or behaved in a way to justify disciplinary proceedings.
The Commission is concerned about the reports in the media and the apparent lack of security that these reports suggest,” it said. “The Commission is of the view that an independent investigation is necessary to ensure public confidence in the civilian oversight of policing in a time of increased armed gardaí in Dublin city in particular.
In a statement, the Policing Authority said it considered the loss of a garda weapon as a “very serious issue” and had raised the matter yesterday morning with Assistant Commissioner for Dublin Pat Leahy, who was in the Authority's office on other business.
The statement said Authority Chair Josephine Feehily had also raised their concerns with acting commissioner Dónall O Cualáin.
“The Commissioner has undertaken to keep the Chair informed of the progress of their investigation into this matter. In addition, the Authority wants to be briefed comprehensively on the operating procedures for carrying firearms. The Authority will also discuss this issue at the next Authority meeting.”
Ms Feehily and Justice Minister Charlie Flanagan commended the civic-minded citizen who handed in the weapon.
Speaking on RTE radio, Mr Flanagan said this ensured that “any danger was averted”. He said the matter was of “great concern” and wondered how it could have happened.
“I'm looking forward to an early report,” he said, “but clearly something went wrong, something happened that shouldn't have happened.”
A Garda spokesperson said: “A senior Garda officer has been appointed to examine all the circumstances surrounding an incident involving an armed Garda unit in Dublin city centre earlier today. Enquiries are ongoing.”