Thousands of euro was stolen from customers in two Bank of Ireland branches when their current accounts were skimmed.
The electronic scam came to light on Sunday evening, prompting account holders in Graiguenamanagh in Co Kilkenny and in Borris in Co Carlow to contact their local branches.
It is understood that Bank of Ireland branches in Roscommon and Mayo were also hit in the past several weeks by the same scam.
Numerous customers had more than €2,000 taken from their accounts, while others had several hundred euro taken out in small amounts over several days.
It is believed the skimming operation took place over three dates in January at the Graiguenamanagh and Borris branches, but that the fraudulent withdrawals only appeared on customer accounts over two weekends.
Sums of €146.13 were taken from accounts over several days and showed up on account statements as $200 being withdrawn from ATMs in the US.
It is thought a small camera was placed at the side of the ATM and an electronic device slipped over where the card was placed into the machine.
It is believed the data was then downloaded by computer and sold on to fraudsters in countries that do not use chip and pin cards, such as the US, where the fraudulent activity was noticed.
A Bank of Ireland spokesperson confirmed that gardaí have been notified.
All of the affected customers will have their accounts reimbursed over the next couple of days and new cards will be issued.
In a statement the bank said: "Bank of Ireland wishes to reassure customers that it is our policy to fully refund any losses sustained as a result of this criminal activity.
"The bank is proactively contacting customers to confirm the fraud and issue replacement cards and will continue to closely monitor the situation, which is also under investigation by the gardaí.
"The bank may also cancel and reissue cards to customers to protect them from fraud if their card details are at risk."