Irish Rail is being criticised for linking to a deals website when customers are buying train tickets online.
Fianna Fáil claims consumers are being duped into paying a monthly charge to a company called Complete Savings.
Other companies such as Ryanair and Ticketmaster also offer their customers the chance to sign up.
Irish Rail spokesman Barry Kenny said customers are warned they will be charged after 30 days.
"Customers who sign up...are explicitly told that a monthly charge applies before they enter in their card details," he said.
"It is completely separate to their booking arrangement with Iarnród Eireann."