More than 600 mobile telephones were seized from inmates in Mountjoy Prison, Dublin, last year, The Irish Times has learned.
Gardaí say they have serious concerns the smuggled phones are being used by prisoners to direct organised crime from their cells.
Evidence has emerged in recent years that a number of gangland killings were ordered and directed from Mountjoy.
The use of phones has become so widespread at the prison that prison officers have been issued with electronic devices to alert them as they walk past a cell that a phone is being used inside.
News of the extent of the problem has emerged just days after five prison officers at three Dublin jails were implicated in a smuggling scam in which telephones, steroids, illicit drugs and alcohol were being smuggled in to inmates.
The officers, four men and a woman, are based in Mountjoy, Wheatfield and Cloverhill.
One from Mountjoy and two from Wheatfield were arrested last week as part of the Garda's smuggling investigation while the other two were spoken to without being arrested.
One of the men had already been suspended from work after a mobile phone, which was not his, was found in his belongings at work.
The Irish Times understands that the suspension from work of the other four officers will be discussed by the Irish Prison Service tomorrow.
Gardaí investigating the matter last week recovered a quantity of controlled drugs and alcohol which they believe were intended to be smuggled to prisoners.
Informed prison sources have told The Irish Times that while just over 600 phones were found in Mountjoy last year they believe many more are in circulation across the prison service.
"We find the phones regularly but we very rarely find the SIM cards," said one source.
"It seems that the prisoners keep the cards hidden and a large number of them can then operate off the one phone by swapping the cards in the handsets. It means they have numbers for everybody they know outside."
While gardaí have long believed prisoners were continuing to organise drug deals from prison, evidence has now emerged of involvement by inmates in gangland murders.
Two prisoners were recently arrested over the murder last November of Latvian mother of two Baiba Saulite (28).
In May 2005, Mark Byrne (31), from Tallaght, Dublin, was shot dead after leaving Mountjoy. It is believed the killer was tipped off via a smuggled phone in the jail.
In July 2005 a criminal trial was told that the killing of Paul Warren (24) in February 2004 in Dublin was, on the night of the killing, directly co-ordinated by a prisoner using a smuggled phone.
Mobiles have for years been thrown to inmates by arrangement over the perimeter walls into prison exercise yards.
Some have also been concealed internally by criminals on committal or by those returning after periods of temporary release. Drugs enter the prisons in the same way.
Once they have been smuggled in phones are concealed in cells and other areas.
Some stashes have been found wrapped in plastic in the U-bends of toilets and down drain holes. However, tiny cameras that can be guided down water pipes and into other inaccessible locations are now being introduced by the prison service to help with searches.