I calculated my monthly mortgage repayments in an excel model and it differed to the amount debiting my account each month so I rang BOI who explained that the difference is probably because the formula they use "is very complicated and is system-based" (?). They sent me a letter with the details of this formula so I re-calculated the monthly repayment and I have been over-charged euro 20 each month for the last 3 years.
I sent my excel calculation to them over a week ago and have not had a response. I advised them that I will cease paying my mortgage until it's resolved.
We had a similar problem many years ago with BoI. We kept track on Excell of interest on a bridging loan while building our house and at the end of the loan we had a much lower figure than the bank. When brought to their attention the answer was basically that they couldn't be wrong & how dare we question it.
Anyway after much arguing we finally presented a print-out of our figures from start to finish & they agreed that they were wrong and refunded the difference.
Persevere & if necessary produce your calculations to them.