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 in Tokyo:

Ireland House 5F, 2-10-7 Kojimachi, Chiyoda-ku, Tokyo 102-0083