Detectives probing the activities of dissident republicans were tonight questioning a man after ammunition was seized in his vehicle.
Armed members of the Emergency Response Unit surrounded the 4X4 as the suspect travelled on the M1 at the border town of Dundalk, Co Louth.
The 33-year-old, who is from Newry, was arrested in an area known as Heynestown at approximately 4.30pm.
He is being quizzed at Drogheda garda station, where he can be held for up to three days.
It is understood the operation was targeting dissident republican activities in the area.
A small quantity of bullets and a balaclava were recovered in the vehicle, which was seized by gardaí.
Forensic officers are examining the SUV for any trace of explosives.