ULSTER Bank has had to refund a large number of mortgage holders after it wrongly calculated the interest they were due to pay on their tracker mortgages.
Some of the people affected told the Irish Independent they were pushed into arrears by the mistakes.
A number of customers had hoped to get a refund before Christmas to ease the financial burden of the festive period.
But Ulster Bank is only now tackling the overcharging.
It comes after Bank of Ireland, AIB and Permanent TSB have all been forced to refund customers due to mistakes in tracker mortgage payments.
A spokeswoman for Ulster Bank said fewer than 500 customers were hit by the overcharging. The Central Bank has been informed and full refunds and back payments of interest would be sent to customers, she added.
Most of the customers affected are understood to have originally taken out tracker mortgages with First Active, which has since been absorbed into Ulster Bank.
Trackers are regarded as great value because they are set at very low levels and can only rise when the European Central Bank rate rises.
The spokeswoman said: "Ulster Bank can confirm that following an internal review, a number of mortgage accounts have been identified whereby an incorrect tracker margin has been applied.
"They will receive a full refund of any overpayment plus compound interest owed and the correct tracker margin will be applied for future repayments."
Karl Deeter, of Irish Mortgage Brokers, said there was an onus on banks to treat customers properly initially rather than having to come back and correct their errors.
Last year Bank of Ireland had to restore more than 2,000 people to trackers after it admitted it failed to observe Central Bank rules about the need to warn customers of the cost implications of giving up a tracker.
The bank paid out compensation of up to €2,000 per customer to 2,096 customers after those who switched from a tracker to a fixed rate were not offered a tracker again when the fixed period was up.
AIB has reimbursed almost €1m after mortgage holders who had originally been on trackers were incorrectly put on standard variable rates when fixed-rate deals ended.
Customers got an average of €4,500 in compensation.
And Permanent TSB was forced to put a number of customers back on trackers last year and compensate them.