A family member passed away recently - what should 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.

Or to either of the Embassy’s offices, in Pretoria or Cape Town:

Embassy of Ireland
P.O. Box 4174
Pretoria 0001
South Africa

or

Embassy of Ireland Liaison Office
19th Floor, LG Building
1 Thibault Square
Cape Town