Mayo University Hospital has apologised to a number of former patients, after confidential medical records were left in a refuse sack in Castlebar.
The personal information related to people recently discharged from the hospital and was found by council staff earlier this month.
Management at MUH has written to all those affected by the data breach, apologising for the incident.
The letter says management acted promptly to retrieve and secure the information, after they were alerted to the find.
The hospital has apologised for the breach of confidentiality.
It says the matter was being treated with the "utmost seriousness" and that corrective measures were in place to prevent such a breach happening again.
The documents contained names, birthdates and other information relating to treatments provided to patients.
