The documents were lost by an AIB worker in a car park in Galway city centre on 31 August.
The material had been gathered ahead of a meeting to discuss what the bank calls "branch portfolios".
Names, balance details, fee data and internal codes were among the information contained on the documents.
It has emerged that the documents were handed in to a business in Galway city shortly after they were lost.
After media reports about the lost documentation a business owner in Galway contacted the bank and told them he had the files.
A member of the public found them in the car park and handed them in to the business in question.
AIB says the recipient had forgotten that he had the documents until he saw the reports about them being missing this week.
The bank has contacted the majority of the affected customers by phone and says it will be issuing letters to all concerned later today.
AIB says all the information has been retrieved and it has informed the Data Protection Commissioner.