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 Passport Office Customer Care:

Consulate General of Ireland
Attn: Passport Section
535 Boylston Street
Boston, MA 02116

Tel: +1-617-267-9330