More than €8.5 million has been left by passengers on Leap cards that have not been used for five years or more.
The figure was released by the National Transport Authority to Sinn Féin Spokesperson on Transport Louis O'Hara.
A Leap card is a reusable plastic card on which credit can be loaded to pay for tickets for public transport in Ireland.
Ten million Leap cards have been sold since they were first introduced in 2011 and the NTA confirmed to Mr O'Hara that there are nearly 2.3 million now classified as inactive.
Deputy O'Hara said: "This extraordinary level of unused credit demonstrates the urgent need for reform in the payment systems used within the public transport system and follows confirmation that contactless payments on public transport may not be operational until 2028."
He called on the NTA to make every effort to ensure commuters can access their unused credit, while also accelerating delivery of alternative payment methods on public transport services in Ireland.
The authority has begun the programme to allow passengers to use contactless payments, such as debit cards, smart phones and other devices.
The system is in use on some local bus services but is not expected to be fully deployed for at least two more years.
