Where to send a deceased person's passport?

You can return their passport for cancellation with a copy of their death certificate to:

Customer Care
Passport Office
Freepost PO Box 9718
Balbriggan,
County Dublin

Or

Passport Office
Embassy of Ireland in Mexico
Cda. Blvd. Manuel Avila Camacho, 76-3
Col. Lomas de Chapultepec Sección I
11000 México D.F. Main Tel +525555205803