A leading dissident republican in his sixties was one of six men being questioned last night after gardaí seized pipe bombs, bomb components, a gun and ammunition.
Twenty searches were carried out in counties Dublin, Louth and Wexford yesterday.
Four men with suspected links to the Real IRA were later arrested in counties Louth and Wexford.
The arrests came as security has been heightened ahead of British royal Prince Charles and his wife Camilla's visit to Ireland next week.
The leading dissident is understood to have several paramilitary convictions, including for directing terrorism.
According to reports, he had close links with the late Real IRA man, Dubliner Alan Ryan, and is close to jailed dissident leader Michael McKevitt.
He was arrested along with three other men in their twenties. All four were being questioned at Garda stations in Dublin last night.
They were detained on suspicion of directing terrorism, membership of an illegal organisation and possession of explosives.
Following the searches, the Irish army bomb disposal team was called in to make safe a device found in Courtown in Co Wexford.
Bomb components were also found during separate searches, including one north of Dundalk in Co Louth.
In a separate development, two men in their twenties were arrested after their car was stopped at a traffic checkpoint at Glenfarne, Co Leitrim, on the N6, Sligo to Blacklion, road at about 11.45am yesterday.
When gardaí stopped the car they found a pistol and two pipe bombs.
The two men are understood to have links to the Continuity IRA in Limerick and organised crime.
Prince Charles and his wife are due to visit the Republic and Northern Ireland on a four-day trip next week.
The British royal is to visit the area in Co Sligo where the IRA killed his great-uncle Lord Mountbatten in a bomb attack.
Lord Mountbatten was killed on August 27, 1979, when a remote-control IRA bomb blew apart his pleasure boat at Mullaghmore where his family often enjoyed holidays.
The 79-year-old - who was Queen Elizabeth's cousin - had a close relationship with Charles.
One of the earl's twin grandsons, Nicholas Knatchbull (14), and a 15-year-old local boy, Paul Maxwell, who worked on the boat, also died in the explosion.
The royal couple are also expected to visit the peace and reconciliation centre at Corrymeela in Co Antrim during their trip to the north.
