Musgrave are advising customers to check their bank statements after a cyber attack.
The company that owns SuperValu, Centra and Daybreak stores says the malicious software tried to extract debit and credit card details from its customers, but claims there is no evidence any data has been stolen.
Gardai and the Office of the Data Protection Commissioner are investigating.
Tech Expert Jess Kelly says it is still not clear when the attack happened.
"The details are still continuing to emerge. Musgrave only flagged this yesterday. It only became public knowledge yesterday, so we think it was in the last number of days," said Ms Kelly
"But the scary thing with malware is that you never really know how long it has been lingering on a system until something catches somebodies eye.
"We don't definitively know as of yet, that's why if you've shopped in any of these stores, I would say over the last month or so, just check your statement to be safe."