Permanent TSB has confirmed it has reported a data breach to the Data Protection Commissioner.
The bank said the breach involved account information relating to less than 100 customers being mistakenly emailed to an unrelated customer.
It is also believed that notes of meetings customers had with bank staff were also included.
It is thought some of these customers were those affected by the recent overcharging controversy.
The bank said the email contained the account number and the names of the customers who held that account together with some other material.
The bank described the information as “limited” and said that neither the addresses nor the contact details of the relevant account holders were sent.
The bank says the relevant customers have been contacted and has apologised to them for the error.
According to PTSB, the bank has been in contact with the customer who received the email and that the customer has co-operated fully with the bank and has confirmed in writing that the email has been deleted from their email account and that they have no copies of it.
In a statement, the Office of the Data Commissioner has said it had received breach reports in relation to a number of minor data protection breaches from PTSB in recent weeks.
It said these are regular routine notifications submitted by the main financial institutions.
The bank says it has conducted a thorough investigation of how this incident occurred and have taken steps to ensure it does not happen again.
Commenting on the breach, Director of the Irish Mortgage Holders' Organisation David Hall said: "It is deeply concerning that such a large data breach of such sensitive information would have taken place by a State owned bank.
"This is further compounded by the fact that a significant number of those affected were customers who were overcharged in the Permanent TSB rates scandal," he added.
