Nine months after extra tax relief was granted to people in the Budget who bought during the boom years of 2004 to 2008, the bank has yet to pass it on to its customers.
It is estimated that those who bought their first homes at the height of the boom could be due to receive tax refunds of up to €500.
The €52m in extra tax relief which will be paid out each year was implemented to provide breathing space for thousands of people who bought homes at record prices.
The increase in mortgage interest relief was supposed to be applied from the start of January for the 270,000 people who bought houses between 2004 and 2008.
However, its implementation was delayed by the need to upgrade the computer systems of the Revenue Commissioners and lenders.
Most lenders are now paying the higher tax relief but Ulster Bank said recently it had still not updated its systems to pay the full amount of the higher relief.
A spokeswoman for the bank said: “We are liaising closely with the Revenue Commissioners to implement TRS changes for our eligible mortgage customers as soon as possible.
“A rate of 25% has been applied to these accounts in the interim period. We will ensure that our customers’ mortgage interest relief is brought fully up to date as the change is implemented.”
The changes in the last Budget mean that people who bought at the height of the boom will receive tax relief of 30% of the interest they are paying on their mortgages – up from 20% before the Budget.
Couples get double the amount of single mortgage holders, meaning higher tax relief is worth up to €2,000 a year for some couples.
Other lenders including AIB, EBS, National Irish Bank, Bank of Scotland (Ireland)/Halifax and Bank of Ireland have applied the new higher 30% rate since May, while Permanent TSB applied it in June with outstanding amounts being spread out in monthly payments until the year’s end.