Have you checked if it is perhaps from a Bank Account that has now become dormant or possibly the old reliable Credit Union, sounds like your looking for a needle in a haystack really. I would imagine if you put your request in writing to any of the banks he had Accounts in requesting that they check all Accounts held in his name, Dormant or Active that had transaction within 6 month period either side of the date of the fund being opened at least it would be a starting point. It would also look like you are making the effort from the Revenue's point of view and who knows they might let it lie, while if you ignore it they will just think you are avoiding it which will just keep them on your heels. Good luck either way.