Army bomb disposal teams were called in after a suspect device and bomb components were discovered in a series of raids.
Four men were arrested by armed gardai after 20 premises across Louth, Dublin and Wexford were searched by detectives probing dissident republicans.
Separately, two men in their 20s were held after a firearm and suspected bomb parts were found in a car on the N16 at Glenfarne, Co Leitrim, at 11.45am.
The seizures and arrests were made as anti-terror gardaí stepped up their activities ahead of a visit to the Republic next week by Britain’s Prince Charles.
Prince Charles and the Duchess of Cornwall will visit the northwest during their four-day trip, which starts on Tuesday.
The pair plan to visit Mullaghmore, Co Sligo, where Prince Charles’s great uncle Lord Mountbatten was assassinated by the IRA in 1979.
"Two males in their 20’s were arrested at the scene and are currently detained under the provisions of Section 30 of the Offences Against the State Act at Manorhamilton Garda Station," said a Garda spokesman about the Leitrim incident.
"The scene is currently preserved and the assistance of the Army EOD has been requested."
Elsewhere gardai said a suspect device was discovered in Courtown, Co Wexford, during an operation targeting dissident republicans.
"The area has been cordoned off and the Army EOD team are on their way to the scene," a Garda spokesman said.
Component parts for explosives devices were discovered at some of the other locations searched.
"All component parts will now be subject to technical examination," he added.
"Investigations are ongoing."
Meanwhile all four suspects are currently detained under Section 30 of the Offences Against Act at various Garda Stations across Dublin.
One of the men is aged in his 60’s and the other three in their 20’s.
The men are being held on several offences, including directing terrorism, membership of an unlawful organisation, possession of explosives.
Gardai said members of the Special Detective Unit on Harcourt Square and Crime & Security in Garda Headquarters were involved int he operation.
Teams were supported by the Garda Emergency Response Unit, Technical Bureau, Dog Unit and uniform and detective Gardai from the Louth and Wexford Divisions.