Eight prisoners have been punished and transferred to other jails after it was discovered they were maintaining Bebo pages from a cell through smuggled mobile phones.
The Mountjoy inmates used at least one mobile phone to send photographs and personal details to the social-networking site.
A search of the Dublin prison's training unit has been carried out by staff but the phones have not yet been found.
The authorities discovered the existence of the pages on Monday and immediately launched an investigation.
Officials said the information placed on the site was relatively harmless, but did involve a serious breach of security in the prison.
They say six prisoners took part in posing for photographs, and believe two others were responsible for organising the incident.
As a result of the investigation, all eight were disciplined and suffered a loss of privileges including restrictions on visits. They were also transferred to prisons with stricter regimes.
Officials said none of those involved was a high-profile prisoner and they were satisfied the Bebo contact was through mobile phones rather than through a prison computer.
This is the third embarrassing incident for prison authorities involving the Bebo website so far this year.
Last month, the website of Limerick prisoner Liam Keane (23) was shut down after authorities discovered his online profile had been updated since he was remanded in custody earlier in the summer.
At least one picture uploaded on to Keane's personal site was apparently taken on a mobile phone in a prison cell.
Keane, who was remanded in custody in May for possession of a loaded Glock pistol, had been sending images of himself to a person outside the prison and his Bebo site was then being updated.
A similar breach of security was discovered in Mountjoy, where a west Dublin prisoner had been urging online users to send messages to his site, which contained photos of himself and other inmates.
Prison officials said last night that they could not guarantee there were no mobile phones in jails, despite regular searches, until new blocking technology had been introduced.
The technology has been operating on a pilot basis at the Midlands jail in Portlaoise and, according to officials, has been working effectively.
It is expected that the installation of the technology in all the country's closed prisons will be completed in about a year.