The Irish Payment Services Organisation has said that ticket vending machines at DART and train stations have been targeted by people attempting to capture bank card details.
It said there have been at least three cases in the last week, including one at Booterstown DART Station and one at Dun Laoghaire train station.
The Garda Bureau of Fraud Investigation is investigating the matter.
IPSO said attempts are being made to ensure that any accounts attached to payment cards that may have been compromised in this way, are monitored for potential fraud spend.
It also warned that this kind of activity is likely to escalate in the run up to Christmas, where a larger number of people than usual are travelling to towns and cities.
IPSO said cardholders using these machines should cover the PIN pad when keying in their number so that criminals cannot see the PIN being entered, either through "shoulder surfing" or when micro-cameras are used.
The banks and IPSO said that they have secured ATMs from skimming attacks, but criminals now are using a new type of skimming equipment at unattended payment machines for the same purpose.