Three teenagers were arrested today after a garda on a regular patrol drove past a petrol station to see staff being held up at gunpoint.
Staff at the Esso station in Kilbarrack cowered as the raiders threatened them with a handgun and took a small amount of cash from the till.
But the armed detective saw the robbery in progress and detained three men before they were taken into custody.
A Garda spokesman said: "We like to call it good policing, but really the detective was passing at the time and caught them red-handed.
"How often do police on a regular patrol stumble across something like this? It was quite incredible.
"There was a gun involved and for one officer to tackle all three men required courage, but that is the way they are trained."
Three teenagers, aged about 18, were being detained at Raheny and Clontarf Garda stations and questioned.
A police spokesman said they were being held under Section 30 of the Offences Against the State Act.
A firearm and a small amount of cash had been recovered, the spokesman said.
PA