The Sunday Tribune recently revealed that Kenny, convicted of armed robbery, began sending the postcards to staff at Mountjoy weeks after he was wrongly released from jail. The 35-year-old was freed on 20 June when another prisoner of the same name, who was serving a sentence for road traffic offences, was granted temporary release.

Kenny, from Ballyfermot, was not due to be released until 2010 but collected his passport and fled the country before gardai were alerted.

In recent weeks he has sent five postcards to friends inside the prison even though he was aware they would be intercepted by staff, who vet all correspondence. He boasted about how much he was enjoying life as a free man away from rainy Ireland. The cards were sent from mainland Spain, Tenerife and Lanzarote.

One media report suggested that prison officers conspired to send postcards while on holiday purporting to be from Kenny in an attempt to embarrass prison management. But detectives have compared the handwriting on the postcards with Mark Kenny's and have concluded they are the same.

One source said: "There was never any doubt in our mind that Kenny was behind this campaign. What was written about prison staff was not credible. All the postcards were written by the same person and we have matched them with samples of Kenny's handwriting."