The chaplain of Mountjoy Jail in Dublin was used unwittingly yesterday in an attempt to smuggle a mobile phone into the jail -- in a birthday cake.

Gardai have interviewed the chaplain, Sister Eithne, but are satisfied that she is innocent and had no idea that a phone was concealed in the cake.

The request for a birthday cake was made to the jail authorities by a prisoner serving a two-year sentence for an arson attack. It was granted, and a senior Mountjoy official asked Sr Eithne to collect the cake and bring it into the prison.

When the chaplain arrived back at the prison yesterday at lunchtime she handed over the cake to be scanned through the x-ray machine that is being used to combat the smuggling of contraband goods into the jail.

The machine immediately detected an object hidden in the cake and prison officers removed a mobile phone.

Prison authorities contacted the gardai, who began an investigation. Initial statements are being taken from the chaplain, the senior prison official and staff at the entrance to the complex. Detectives will also interview the prisoner, and further inquiries are being focused on the source of the cake.

A full report on the incident was being awaited last night by the director general of the Irish Prison Service, Brian Purcell.

A spokesman for the Prison Service said last night that the incident underlined the effectiveness of the recently installed security equipment.

Since the x-ray machines were introduced, the smuggling of drugs and mobile phones into Mountjoy through visitors has ceased and the price of purchasing a phone in the jail has jumped dramatically.

Officials said this incident showed prisoners were becoming desperate to find other ways to bring in the contraband.

"This is a classic case," one official said last night. "It's like a plot out of an old English film, made in Pinewood studios. But in this case, heavenly intervention was on our side."

It is a criminal offence to smuggle a mobile phone into a jail, carrying a maximum sentence of five years in prison.