National Irish Bank has begun to reimburse customers whom it overcharged during a 10-year period leading up to 1998.
The bank said it was automatically reimbursing customers who were overcharged by less than €13 during the period and was individually assessing all cases involving more than €13.
The overcharging relates to manual amendments to fees and interest that were initially calculated by the bank's computer system.
A High Court investigation found that the controls governing the way such charges were applied were inadequate.
The bank sent letters to 47,000 customers today advising them of the reimbursement programme.
It has also set aside €10.6m to cover the total cost of the programme.