Northern Ireland police say they have arrested four suspected IRA dissidents and seized a handgun in their car, while Irish army explosives experts pored through the contents of a dissident arms bunker south of the border.
Tuesday's moves are the latest by police in both parts of Ireland to suppress Irish Republican Army splinter groups plotting bomb attacks in Northern Ireland.
Northern Ireland police declined to identify the four men arrested in the same car near Cookstown.
Irish army officers blew apart a homemade mortar shell found in the arms bunker. The IRA typically uses such truck-mounted mortars against fortified security bases.
The arsenal uncovered in County Louth, near the Northern Ireland border, included TNT explosive and a heavy machine gun.