Four men are being questioned by Gardai in connection with an investigation into dissident republican activity.
It's understood a leading figure in the republican movement is being detained after officers carried out more than 20 searches in three counties.
The swoop is part of on overall increase in activities by anti-terrorism Gardaí ahead of the visit of Britain's Prince Charles next week.
Three of the men arrested are aged in their 20's, while the fourth is in his 60's.
They're being held over alleged offences including directing terrorism, membership of an unlawful organisation, and possession of explosives.
