Four men have been arrested in connection with an improvised mortar and launch tube that were found in a car in Dundalk last night.
Last night's arrests were part of an ongoing operation targeting dissident republicans.
An Army bomb disposal team was called in to make the device safe after it was discovered in a vehicle that had been forced to pull over by gardaí at about 9.30pm between junction 16 and junction 17 on the M1.
It took over four hours in freezing temperatures to make the device safe, and gardaí were forced to close off the M1 in both directions as the mortar could have posed a significant threat to the public.
The scene was declared safe at 2.30am.
Two men aged in their 20s were arrested at the scene, while another man aged in his 30s was arrested nearby.
In a follow-up operation a property was searched in Drumiskin, and another man in his 30s was arrested.
A number of items were seized in this search and are being technically examined.