What to do if I find a passport I'd 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.

You should return the passport to the Passport Section of the Irish Embassy in Paris:

Embassy of Ireland/Ambassade d'Irlande, Passport Office, 4 Rue Rude, 75116 Paris.