Belfast, Northern Ireland (CNN) -- Police in the Republic of Ireland arrested four men after finding a bomb in a car near the border with Northern Ireland, authorities said Wednesday.
Police said the viable mortar with a launch tube was discovered when they stopped the car near Dundalk on one of Ireland's busiest motorways late Tuesday night.
Irish media report the intelligence-led operation involved officers investigating dissident republican activity.
The bomb was intercepted as the car was headed north, raising fears the device was primed for an attack in Northern Ireland.
The M1 motorway was closed while an Irish army team carried out what the army described as "a technically difficult task that took over four hours in freezing conditions," Capt. Pat O'Connor said.
The device was removed for further examination, and the scene was declared safe Wednesday morning.