MOBILE phones in Ireland's highest-security prison are worth more than any other contraband product in the jail and are changing hands for up to €1,500, prison sources said.

A crackdown on their use at Portlaoise Prison has seen a huge rise in the black-market value of cell phones.

The almost total shutdown of the illegal prison network has also had consequences for gardaí, who used to routinely intercept communications from the jail in an attempt to gather intelligence on drug gangs.

"It would be safe to say that this type of intelligence led to a number of drug seizures and stopped quite a number of armed robberies and shootings from taking place," said a garda source.

A prison officer at Portlaoise said that mobile phones "are considered the most valuable product in the jail but nobody would dare attempt to smuggle one of them in due to the current security restrictions.

"Prison officers have been let known that they will be paid handsomely if they could bring one into the jail but the risks are enormous."

Major new security precautions have been put in place since the convicted armed robber John Daly was able to call RTé Radio's Liveline from his prison cell two years ago.

The Irish Prison Service is to introduce new 'detection' chairs for visitors as the latest figures show that more than 100 mobile phones are making their way into the prison each month.

The Body Orifice Security Scanner chairs work by detecting metal objects, which may be hidden inside the body. One of the most common ways of smuggling a phone into the prison is by inserting the device internally.

A total of 2,047 mobile phones were seized during the course of 2008 while 431 phones were found during the first two-and-a-half months of this year. The Department of Justice said that "a large percentage of these seizures are not directly from prisoners but are retrieved at entry point".

The prison service has also introduced mobile-phone blocking technology in a number of jails but thick walls in some of the country's older institutions has proved a severe difficulty, particularly at Mountjoy Prison.