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 at the following address:
Consulate General of Ireland
345 Park Avenue, 17th floor
New York, NY 10154
If you happen to find an Irish passport that someone else has lost, you may return it to the Consulate at the address above.