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 Luxembourg:

Embassy of Ireland, Passport Section, 28, route d’Arlon L-1140 Luxembourg