Royal Bank of Scotland's Ulster Bank says its costs resulting from extensive group-wide computer glitches earlier this summer will grow by "tens of millions of euros" on top of the 35 million euros (£28 million) already set aside.
The computer problems in June and July led to salaries not being paid into accounts, customers not being able to use ATMs, and direct debits not being paid.
Ulster Bank has announced details of a compensation scheme for customers ranging from 25 euros to 120 euros to cover inconvenience, costs related to extra trips to the bank to complete transactions, and charges levied on customers as a result of defaulted payments.
"We set aside a provision of 35 million euros in the second-quarter financial results. These measures will end up costing us tens of millions on top of that," Ulster Bank chief executive Jim Brown told Irish broadcaster RTE.
Brown and RBS chief executive Stephen Hester have waived their bonuses for this year as a result of the fiasco.
RBS revealed in its half year results last month that it will suffer group losses of at least £125 million as a result of the IT failures.
Last month RBS' chief executive Stephen Hester said that the banking group may have avoided the major IT glitch if it had focused more on keeping its existing systems up-to-date, rather than developing new systems.
"RBS has seen a big mushrooming in spending on technology. With hindsight maybe a bit more of that increase in spend should have been in the core, taken-for-granted systems that work every day," said Hester.
"Some of our focus was on the new things people want."