Finally, I would ask AIB formally why the SO was not automatically cancelled after it was failed a number of times. I worked on the SEPA implementation for a company and the business rule that was suggested we implement by the bank was if we got back an error stating "account closed" or "account does not exist" the Direct Debit is automatically cancelled. It seems odd they would continue to process it after it failed for that reason. For reasons like insufficient funds you would only cancel it if it failed 3 times in a row - for example.
Also did AIB ever alert you to the fact the SO was failing ?
The op knew the so was failing because it was returned to the op's account every month. Also I am not sure if AIB have the authority to cancel a standing order initiated by one of their customers?