Commuters and Christmas shoppers are being targeted by a criminal gang skimming bank cards at ticket vending machines.
Devices have been uncovered in Dart and railway stations in south Dublin in recent weeks, the Irish Payment Services Organisation (IPSO) warned.
The Garda Bureau of Fraud Investigations (GBFI) is investigating the cases and recovered CCTV, which suggests a single gang is operating the scam.
The IPSO said that while banks in Ireland have secured ATMs from skimming attacks, criminals are now using a new type of skimming equipment at unattended payment machines.
Criminals are capturing debit and credit card details and Pin numbers when unsuspecting cardholders are using the machines, it said.
It warned: "There have been at least three known cases in the last week, including an incident at Booterstown Dart station and one at Dun Laoghaire train station.
"IPSO has been working with the machine operators and the card issuers to ensure that any accounts attached to payment cards that may have been compromised in this way are monitored for potential fraud spend.
"While this kind of activity will most likely escalate in the run-up to Christmas, where a larger number of people than usual are travelling to towns and cities, IPSO warns cardholders using these machines to simply cover the Pin pad when keying in their Pin so that the criminals cannot see the Pin being entered, either through shoulder-surfing or when micro-cameras are used."
Cardholders who believe they may have been compromised should check their accounts on a regular basis.
Bank statements are available 24 hours a day on the internet or over the phone and any unauthorised cash withdrawals or sales should be reported immediately to the card-issuing bank and gardai.