A Mountjoy inmate was hospitalised after being beaten up by fellow prisoners when he was caught in possession of €21,000 worth of heroin that was to be distributed in the jail.

The 33-year-old, who is serving six years for armed robbery, was given day release last Wednesday on compassionate grounds.

When he returned to the prison on Wednesday evening he was searched by staff and found to be in possession of a significant amount of heroin, estimated to be in the region of €21,000.

Gardaí were immediately called and the prisoner, who is from Dublin's north inner city, was arrested and taken away for questioning to the nearby Mountjoy garda station.

He was returned to prison the following morning, when prison staff noticed a disturbance in his cell and saw him being assaulted by a group of men.

The fight was broken up and he was taken to the Mater Hospital and treated for a hand injury. It later emerged he had suffered a broken finger.

Prison sources say several prisoners clubbed money together to buy the drugs, which were to be broken down into individual deals and sold for profit at the jail. The man was beaten up by his would-be drug-dealing partners when the haul was discovered. However, new security procedures in all prisons, including airport X-ray machines, meant that the plot was foiled.

After Wednesday's incident, prison staff searched a cell at Mountjoy and discovered around €1,000 worth of heroin. On Thursday a woman went to visit her partner in the jail and a sniffer dog detected she was carrying drugs.

Gardaí were called and €25,000 worth of heroin was discovered. She was arrested and is expected to be charged.

A prison spokesman said the three incidents last week show that the new security measures are working.