Five men are being questioned in the Irish Republic over suspected dissident republican activity.
The arrests were made in Co Louth after gardai stopped and searched two cars.
The first vehicle was pulled over close to the border on Ecco Road in Dundalk town.
Officers from the Emergency Response Unit (ERU) found a firearm and balaclava during the search, and two men were arrested at the scene.
The pair, in their mid 20s and mid 30s, are currently being held at Dundalk Garda Station under Section 30 of the Offences Against the State Act.
Shortly afterwards, three men were arrested when a second car was stopped and searched in Ardee.
ERU officers also discovered ammunition and a balaclava in the vehicle.
The men, who range in age from early 30s to mid 50s, were taken to Drogheda and Balbriggan Garda Stations.
Local gardai also assisted in the operation, which was part of an ongoing investigation into dissident republican activity.