An Post Money Currency Cards have been suspended until further notice after the issuer of the card, collapsed German firm Wirecard, was required to cease all regulated activities by the UK financial regulator.
In a statement, An Post said this means the cards cannot be used in Ireland or abroad.
It said there are around 50,000 cards in circulation but cannot confirm the amount of money on them.
The card is a prepaid MasterCard that allowed customers to purchase 16 different currencies.
An Post said it "regrets this inconvenience to our customers and we are working to ascertain what impact this situation will have on customers who hold an An Post Money Currency Card".
The Financial Conduct Authority in the UK imposed a number of measures on the issuer of the card, Wirecard Card Solutions Limited, because "there are ongoing events in Germany concerning companies closely linked to Wirecard."
"Wirecard's parent company, Wirecard AG based in Germany is currently the subject of law enforcement interest and insolvency proceedings".
Yesterday Wirecard filed for insolvency after a €1.9bn accounting scandal emerged at the firm.