The details are included in a report by the Central Bank into the use of promotional interest rates. It did not name the banks examined.
The report, which covered six banks, found several errors in bank products with promotional interest rates.
It included five deposit and six home-loan products which have been used by a total of 112,000 customers.
Over 200 customer and 40 complaint files were reviewed. The Central Bank said that refunds will be paid to customers as a result of the investigation.
In the case of 'several of the homeloan products', customers were not given the full period at the promotional interest rate.
The Central Bank says that fixed rate, homeloan customers were also affected, although they were not included in the inspection.
One bank is refunding affected customers, while an investigation is ongoing at one bank to rectify the issue.
Two brochures for one deposit product were found to be 'unfair and misleading' to customers.
The relevant bank is now in the process of refunding those customers.
The inspection also found that two banks had difficulty locating signed application forms submitted by customers.
The Central Bank also reveals that it is particularly concerned that IT systems did not deliver the terms and conditions of the products examined, despite the fact that it has previously raised this issue with the banks concerned.
A spokesperson for Bank of Ireland said it was not one of the banks mentioned in the Central Bank release.
A spokesman for AIB said that where customers are affected, they will be refunded with compensatory interest.
He also said the bank apologised for any possible errors and said the bank would thoroughly go through the Central Bank findings.
Permanent TSB said it would not be making any comment.
