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 Spain you should return the passport to:
Passport Officer, Embassy of Ireland, Paseo de la Castellana, 46 - 4, 28046 Madrid, España