What do I 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 Embassy of Ireland, 3 Ritan Dong Lu, Beijing, China
爱尔兰大使馆 | 中国北京市朝阳区日坛东路三号 100600