Gardaí carrying out speed checks in Co Monaghan stopped a man carrying four handguns as well as ammunition on Wednesday afternoon.
Officers from the Carrickmacross Roads Policing Unit detected a car speeding on the N2 in the townland of Lisanisk.
The car was pulled over and the driver, a man in his 30s, was asked to get out for a roadside drugs test.
Following the test gardaí searched the car and found four pistols and ammunition.
The man was arrested and is currently detained at Carrickmacross Garda station under anti-terrorism legislation. He can be questioned for up to 48 hours.
A Garda spokeswoman said investigations are continuing.
A source said detectives believe the man may be associated with a dissident republican group but stressed that investigations are at an early stage.
