GARDAI have foiled a terrorist plot to intimidate and harass members of the Special Branch and their families.
Officers believe the Real IRA, whose leader Alan Ryan was murdered earlier this month, were behind the spy plan.
The dissidents planned to gather intelligence on key detectives and compile a dossier on their homes, families and leisure activities.
But the plot fell apart when gardai launched a counter-surveillance operation in Dublin and arrested two suspects.
The Real IRA surveillance began earlier this week when a man using a false name rented a room in the Harcourt Hotel, directly across the road from the Garda's Dublin headquarters at Harcourt Square.
The man asked hotel staff for a room looking out on to the garda complex, which houses the Special Branch, the National Bureau of Criminal Investigation, the National Fraud Bureau, and the Criminal Assets Bureau.
The room was to be used by the dissidents for spying on the movements of known members of the Special Branch as they left their headquarters.
Gardai believe it was the start of an intelligence gathering operation, which was due to be developed extensively after they had gathered details of the garda movements and the registration plates and descriptions of the vehicles used by the detectives.
The initial information would then form part a key part of the plan to keep watch on the anti-terrorist officers as they travelled to their homes and expand the dossier on their private lives.
Gardai became suspicious when they spotted the main figure of the plot in the area and identified him as an associate of Alan Ryan.
A counter-surveillance operation was set up and officers moved in on Wednesday evening. They arrested the 41-year-old dissident and another suspect, a 32-year-old man from Cabra, in Grafton Street.
The second suspect was released without charge last night.
When officers raided the hotel bedroom they seized a digital camera, which was examined yesterday and found to contain photographs of several gardai as they walked in and out of the Harcourt Square complex as well as other footage.
Gardai also found a substance, which could be used by the photographer to blacken his face while leaning out of the bedroom window, dark material for wrapping around his neck, and baseball caps.
The chief suspect was being held last night at Irishtown garda station under section 30 of the Offences Against the State Act, which allows gardai to hold him without charge for up to three days.
He is also being questioned about alleged membership of an unlawful organisation and providing assistance to an unlawful organisation.
A detailed forensic examination of the room was continuing last night as the Special Branch extended their inquiries into the spying operation.
Detectives have now been advised to take extra precautions about their personal safety and senior officers say they are determined to press ahead with their crackdown on all of the dissident groups.