The company that owns SuperValu, Centra and Daybreak stores says it's been hit by a cybercrime attack.
Musgrave says the malicious software tried to extract debit and credit card numbers and expiry dates of its customers, but says there is no evidence any data has been stolen.
The group is apologising to customers and advising anyone concerned to check their statements.
Gardai and the Office of the Data Protection Commissioner are investigating.
Tech Expert Jess Kelly says it can be tricky to find out who is behind it "you have hackers sitting in their bedrooms in parts of this country, and abroad. We are going to see more and more of these attacks".
"Businesses are quite vulnerable, if you don't have some sort of protection in place now is the time to do it. When it impacts your business, it's too late", she warned.