Detectives have been investigating suspected collusion by staff in bringing phones and drugs into the prison and supplying them to inmates.

Several of the five suspects have admitted receiving sums from 100 to 600 in return for supplying prisoners with mobiles. The Irish Prison Service has launched a major crackdown on the availability of phones behind bars.

Last March, three prison officers were arrested on suspicion of smuggling phones and drugs into three Dublin jails and the lockers of several other staff members were searched.

One prison officer gave gardai a detailed picture of how he sneaked phones, steroids and alcohol into work and then sold them. He had a serious cocaine habit and needed ready access to cash. He has since been suspended.

Prison management has been aware for some time that a small number of rogue staff have been involved in the sale of contraband and have been working in cooperation with detectives from the National Bureau of Criminal Investigation.

As a result, several measures have been introduced, including sniffer dogs, X-ray machines and devices to block mobile phone signals.

Nearly 1,400 handsets were seized in the first seven months of the year and 12 men appeared in court last week charged with illegally possessing mobile phones. They face the possibility of a 10,000 fine or a fiveyear prison term for the offence.

A source said: "It is simply unacceptable that a small number of staff have been colluding with prisoners to get them mobile phones. In many cases prisoners have been organising crimes from their cells.

"We cannot turn a blind eye to this. If a member of staff takes in a mobile phone and gets paid for it, what is to say that a knife or a gun won't be next?

"They also open themselves up to blackmail at the hands of prisoners. We know that it is going on but are determined to stop it. If the DPP decides to charge staff then that will send out the strongest possible message."

Garda sources say investigating the spate of incidents of phones being recovered in cells is taking up a significant amount of their time.

A total of 107 phones have been seized in Mountjoy in the past six weeks. Gardai are routinely informed of each seizure and have to arrest and question suspects, which they say is very time-consuming.

Detectives at Mountjoy garda station say they are spending more time investigating prison-related offences than ordinary crime in their district. This is despite the fact that there are two dedicated detectives working on prison enquiries.

Ten detectives from Mountjoy are now involved in quizzing prisoners inside the nearby jail.