Gardai hoped to make up to a dozen arrests during the operation which began at 10am yesterday and lasted 12 hours.

By 3.30pm, over 20 searches had been carried out and seven people had been detained.

Four were arrested on suspicion of robbery after three people were mugged in separate incidents. Two detectives adjacent to the prison witnessed one of the robberies and made the arrests.

Prison management and senior gardai are growing concerned that larges amounts of contraband are being thrown over the walls of the prison on Saturdays for collection by inmates.

Detectives also launched several intelligence-driven searches of individuals visiting the jail who are suspected of passing drugs to inmates. Prison officers inside the control room at Mountjoy used camera equipment to record anything that was thrown into the prison grounds and to film prisoners who went to collect the material.

The operation is the latest in a series of initiatives cracking down on the widespread availability of phones, drugs and alcohol inside Ireland's oldest jail.

A total of 107 mobile phones have been seized at the jail over the past eight weeks and dozens of prisoners are facing prosecution with the possibility of an extra five years being added to their sentences.

One garda source said yesterday: "We have around 20 gardai all in plain clothes in and around the perimeter of the jail. Saturdays are terrible for people walking up to the walls of the prison and throwing bags with drugs, alcohol and phones over into the grounds.

"The prisoners know exactly when the stuff is due to arrive and take it before the guards have a chance to react. It has become so bad that people climb over walls into the back gardens of local residents so they have a better throw. We have put a lot of planning into this operation and have a list of names of people we will search when they present themselves for their weekly visit."

Yesterday's operation was led by Inspector Mick Connell of Mountjoy garda station and involved detectives and gardai from Mountjoy and Fitzgibbon Street. The head of the prison service, Brian Purcell, has made cleaning up the country's jails his main priority.