A man in 40s was detained and a rifle found after a car was stopped and searched on the Point Road, Dundalk on Tuesday.
A man in his late 50s was later arrested during a follow-up search at a house also in Dundalk.
GardaÃ­ described the original search of the car as a planned operation. Both men remain in custody.
Two men have been arrested in County Louth by police investigating dissident republican activity.