The National Treasury Management Agency was overcharged €3.2 million by financial services firm State Street.
The agency said it had since been reimbursed by the firm, which provided transition management services for the National Pension Reserve Fund.
State Street has been fined £22.8m by Britain's Financial Conduct Authority for systematically overcharging six clients, to the sum of $20.17m.
According to a statement on the FCA website, State Street’s British transition management business had “developed and executed a deliberate strategy to charge clients substantial mark-ups on certain transactions”.
In a statement the NTMA noted the FCA fine and said it would study its notice before deciding whether to take further action.
State Street's overcharging came to light after a client of the firm identified mark-ups on certain trades that had not previously been agreed.
Initially staff in the firm claimed the issue was an inadvertent error, the FCA said, and while they agreed to arrange a rebate, they did not disclose instances of overcharging in other trades.
An investigation found that the firm had breached three of the FCA’s principles of business and, following a probe by senior management at State Street UK, steps were taken to improve the company’s processes and systems.
State Street agreed to settle at any early stage in the FCA investigation, which qualified it for a 30% discount on the fine imposed.
In a statement today, State Street said that it "deeply" regretted the matter. It said that over the past several years, it had worked hard to enhance its controls to "address this unacceptable situation".
State Street said that it dismissed individuals centrally involved in the overcharging of transition management clients in 2011. "Their behavior was unacceptable and a significant departure from the high standards of conduct and transparency that we expect and certainly not consistent with the manner in which our employees act on behalf of clients every day," today's statement said.
"We are confident that we have addressed the weaknesses highlighted in the FCA’s notice and as a result, have emerged as a stronger organisation," the statement concluded.