Large enough orgs, and development NGOs, just use coordinates and maps. Like they always did. Because even a service like W3W needs to know that a place exists in order to name it. Once someone knows the place exists, and where, you can just put it on a map. Problem solved.
Regarding general addresses, Ireland has no ZIP codes (at least until 2017, last time I had to worry about that). Their postal service works just fine.
I was thinking that some of these orgs could collaborate on an open standard that serves the same purposes served by What3Words, for the people for whom it serves those purposes. But with an open standard that doesn't sound overly proprietary.
Regarding general addresses, Ireland has no ZIP codes (at least until 2017, last time I had to worry about that). Their postal service works just fine.