Gardai seized an improvised firearm when they arrested a 42-year-old man for drink driving on the M50 yesterday.
The weapon was loaded and capable of firing .22 ammunition.
The incident unfolded when officers from the Dublin traffic corps responded to a minor traffic collision on the M50 northbound at the N4 junction at 11.30am.
No one was injured in the crash.
When they got there, officers suspected that the driver of the car had been drinking and he was arrested.
They then searched the car and discovered three other rounds of ammunition and what is described as an "improvised firearm".
The suspect, who is known to gardai, was brought to Ballyfermot Garda Station where he was still being questioned today.
He is being held under section 30 of the Offences Against the State Act.
Senior gardai have praised the work at the scene of the officers from the regional traffic unit who are based at Dublin Castle.
"These officers were very proactive in how they dealt with the situation. They realised that it was more than possible that the individual might have been in possession of illegal items and so it has proved," a garda source said. It is understood that the traffic corps have been involved in a number of probes against criminals.