The European Central Bank (ECB) has fined AIB and its subsidiary EBS a total of €615,000 for miscalculating their capital needs.
The errors occurred between 2014 and 2016, the regulator said.
AIB must pay an administrative penalty of €420,000 for reporting lower Risk Weighted Assets (RWAs) for intragroup equity exposures than they should have over nine quarters during the period.
EBS received a fine of €195,000 for similar mistakes over seven quarters.
The ECB said it classified the breaches as moderately severe.
RWAs measure the level of risk that a bank carries on its balance sheets and enable lenders to work out how much capital they require.
By not calculating the necessary level of capital properly, the ECB said AIB and EBS reported too high a figure for a key indicator of their ability to absorb losses, known as Common Equity Tier 1 ratio (CET1).
However, because the errors related to equity exposures at an individual bank level, they had no impact on the overall group CET1.
In a statement AIB Group said it informed the ECB that it had misinterpreted certain provisions of the Capital Requirements Regulation in AIB and EBS in the period from March 2014 to March 2016.
"As soon as the issue was identified, we brought it to the attention of the regulator, restored compliance and co-operated fully with the ECB review," it said.
"The breach had no impact on customers and no impact on the Group's consolidated capital ratios. We have since strengthened our controls to prevent any recurrence."
The lenders are able to challenge the ECB decision before the European Court, but a spokesman for AIB Group said it would not be appealing.