Four men are being questioned after a mortar bomb was seized in a sting operation on a suspected dissident republican gang.
Army bomb experts worked overnight in freezing temperatures to make safe the improvised grenade launcher on the M1 motorway near the border.
The rocket launcher was found in a car which gardai stopped and searched as it travelled north between junction 16 and junction 17 near Dundalk.
The improvised mortar and launch tube, described as viable, was made safe by the Army bomb squad and the motorway was closed for several hours.
A Garda spokesman said two men were arrested at the scene, both in their 20s, and another man in his 30s was detained nearby.
In a follow-up operation, a property was searched in Drumiskin, Co Louth, and another man in his 30s was arrested.
The spokesman said a number of items were seized in the second operation which were taken away for technical examination.
The four suspects were detained in Drogheda, Balbriggan and Santry Garda stations under section 30 of the Offences Against the State Act.
A Defence Forces spokesman said the operation to make the mortar safe took more than four hours amid temperatures as low as minus 6C and was technically difficult.
The motorway between Dublin and Belfast was shut down in both directions for several hours, while bomb experts said the homemade device "posed a significant threat to the public". It was removed for further analysis and the scene was declared safe.