A third man is being questioned over a suspected dissident republican spy operation on Dublin's Garda headquarters.
The 23-year-old was detained in Ballymun, in the north of the city.
He is being held under Section 30 of the Offences against the State Act.
The arrest is linked to an operation started in September, when two men were taken into custody after surveillance equipment was seized in a hotel room overlooking Harcourt Square Garda station.
The regional headquarters is home to a number of the force's specialist units, including the Special Detective Unit, which investigates the activities of dissident republicans.
It also houses the National Bureau of Criminal Investigation, Organised Crime Unit, Emergency Response Unit and Garda Bureau of Fraud Investigation.
Detectives became aware of suspicious activity near the station when a suspect known to them was observed at the nearby Harcourt Hotel.
The person rented a room at the hotel overlooking the garda station using a false name.
Two men, aged 41 and 32, were later arrested on nearby Grafton Street. The 41-year-old appeared before the Special Criminal Court while the 32-year-old was released without charge, pending a report to the Director of Public Prosecutions.
The third man was later released without charge. A Garda spokesman said: "A file will be prepared for the Director of Public Prosecutions."