RAIL-users will be hit with a backdated fee on Monday after some payment cards failed to register a charge.
More than 9,000 individual cards were not charged for tickets bought at ticket vending machines (TVMs) over a period of 10 weeks.
Iarnrod Eireann has apologised for any inconvenience but said that the error occurred during a recent software upgrade.
However Thomas Stamp, of representative group Rail Users Ireland, said it was "unfair" to hit customers with the fee so many weeks after they believed they had paid.
"It is not passengers' fault. They used the service in good faith," he told the Herald.
The problem affected certain payment cards, mainly AIB Maestro Debit cards.
Purchases made with these cards at TVMs from March 28 to May 31 inclusive were affected.
"A fault developed which saw tickets issued and payments authorised against certain payment cards," a company statement said. "However, the payment was not subsequently deducted from customer accounts."
All affected cards with one single transaction amounting to less than €50 will be processed on Monday.
The company said that it recognised that processing cumulative payments at one time may cause difficulties for some customers.
And instead of charging one lump fee, it said it would withdraw payments from customers' bank accounts to match the same intervals between original transactions.
Multiple transactions which exceed over €50 will be charged on a phased basis – this will also begin on Monday.
"This fault has now been rectified," Irish Rail added.
"TVMs are now charging correctly against all cards."
But it said that customers could not have been overcharged in this instance.
"The technical issue caused payments not to be processed on certain payment cards only," the statement outlined.
"No overcharging occurred."
But Mr Stamp said that the move was "counter-productive".
"It will make people think twice about using the service.
"It could affect passengers by putting them into overdraft or beyond. It is totally and utterly wrong."