A family member passed away recently - what do I do with their 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,
Ireland

or to the Consulate at

Passport Office
Consulate General of Ireland
345 Park Avenue, 17th Floor
New York, NY 10154