The Irish Payment Services Organisation is reassuring bank customers that they will not be liable for fraudulent transactions on their credit cards.
Hundreds of customers have been hit by an online scam.
It is believed the credit card details were accessed several months ago, but have only begun to use them in the past two days.
The monitoring system operated by Irish banks spotted the unusual transactions, after the card details were used to buy low-value items, for €1 or €2, on a US website.
It is thought the criminals were testing the cards, to check if they were still valid.
