Two men were questioned this evening after officers from the Special Detective Unit appeared to foil an attempt to spy on An Garda Síochána's Dublin Headquarters in the city centre.

A 41-year-old man remains in custody, while a 32-year-old man has been released. A file is being prepared for the Director of Public Prosecutions.

Detectives investigating the activities of dissident republicans searched a room in the Harcourt Hotel, near the garda headquarters in Dublin, and found surveillance equipment, including a digital camera.

Images taken from the seized camera were of some serving officers, as part of what detectives believe was an attempt to gather information about gardaí.

The hotel is across the road from the building housing the garda specialist units, the National Bureau of Criminal Investigation, the Organised Crime Unit, the Garda Bureau of Fraud Investigation and the Special Detective Unit.

The seizure was made after a known dissident republican was seen in the area yesterday afternoon, raising garda suspicion.

He was tracked to a room in the hotel and when gardaí discovered it was booked in a false name they searched it and found surveillance equipment, including the digital camera.

He was subsequently arrested on Grafton Street in Dublin, along with another man.

This is not the first time that a subversive group has tried to spy on gardaí.

More than 20 years ago an IRA man turned Sinn Féin advisor was found taking pictures from an attic in a house across the road.

He was subsequently caught with firearms and sentenced to eight years in prison.

Dissident republican groups have found themselves the subject of an intense investigation following criminal activity at the funeral of Alan Ryan earlier this month.

Over 20 people have been arrested, three have been charged and a man is still being questioned today after a gun linked to a gangland shooting was found in Tallaght yesterday.