Tens of thousands of workers are due a refund for overpaying their PRSI.
The Department of Social Protection has confirmed that €10m has already been paid back to taxpayers, who have received an average refund of €400.
The overcharging is due to the way the health levy element of social insurance was applied in 2010.
Anyone earning less than €26,000 was not liable to pay the levy.
However, if a person earned more than €500 in a given week the levy was automatically deducted from their pay.
This means those that earned over €500 a week - but less than the annual threshold of €26,000 - may be due money back.
The Department of Social Protection insisted this is not an error and it said the system was set up to deduct PRSI on a weekly basis and workers could later apply to get the money refunded.
However, the refund is not automatic and the Consumers' Association of Ireland is advising people to check their payslips and apply to the Department if they think they are due money back.
It is not yet known how many people may be entitled to claim.
The address for refunds is: Department of Social Protection, PRSI Refunds Section, Oisin House, Pearse Street, Dublin 2.