The Irish Prison Service says it first found out about the inadvertent recording of prisoners phone calls to solicitors last November.
But it says the full extent of the problems only came to light last Wednesday when it ordered a full security audit.
It says 81 calls made by prisoners were accessed by prison staff.
The Prison Service is legally required to record all calls made by prisoners - except those to their solicitor and to the Samaritans.
Just as with the Gardaí, it says it first became aware of an issue about inadvertent recordings of phone calls with solicitors last November.
But it says the full extent only emerged last Wednesday, after an audit carried out in response to the scandal of the Garda Tapes.
Yesterday it told the government about 84 prisoners who were affected by an issue where their second solicitor was not on their Prison Service record - and whose calls were therefore recorded.
Last night it said it had identified a further 55 prisoners affected by the problem, and that it will write to each affected prisoner to apologise.
Over 2,800 calls were recorded - 81 of which were accessed by prison staff - but the agency insists that NO contents of any calls were shared with a third party.
