Three men with links to dissident republicans are being questioned after a machine gun and a pistol were seized by gardai in the Midlands.
Detectives are investigating if the weapons were transported across the Border from the North recently and being held at a house in Longford before being recovered by gardai.
The operation began yesterday morning when heavily armed gardai from the Emergency Response Unit (ERU) stopped and searched a car travelling through the Drumlish area of the county.
During a search of the vehicle ammunition was seized and the driver, who is aged in his 40s, was arrested.
Follow-up raids involving the Special Detective Unit (SDU) were carried out at a nearby house where two deadly firearms - a machine gun and a pistol - were discovered.
Two men aged in their 30s and 40s were arrested at the scene and taken to Longford and Granard garda stations.
All three men are being quizzed under the Offences Against the State Act and can be held for up to seven days.
A source said the men had links to dissidents, and gardai were attempting to establish what the weapons were being used for.
"The men, who are from the area, have links to dissidents and were arrested following a lengthy surveillance operation," the source said.
"What exactly these guns were being used for is still under investigation, but the situation is clearly sinister and the weapons appear to be in good condition."
A garda spokeswoman said the searches were part of an "ongoing operation by garda security and intelligence including members of the Special Detective Unit".
Local gardai from the Granard district also assisted the national unit with the operation.