Five men were being questioned in the Republic last night over suspected dissident republican activity.
The arrests were made in Co Louth after gardai stopped and searched two cars yesterday.
The first vehicle was pulled over close to the border on Ecco Road in Dundalk town at around 11.25am.
Officers 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.
Shortly afterwards three men were arrested when a second car was stopped and searched in Ardee.
Officers also found ammunition and a balaclava in the vehicle.
The men, aged from early 30s to mid 50s, were taken to Drogheda and Balbriggan Garda stations.