What should I do if I find a passport I'd previously reported lost/stolen?
Never travel on your passport if it has been reported lost or stolen. You run the risk of being stopped at border crossings, as Interpol keeps a record of all reports of lost or stolen passports.
If living in Greece, you should return the passport to the Passport Section of the Irish Embassy in Athens:
7, Leof. Vas. Konstantinou, Athens 106 74.