Here's a lovely wall-embedded ER to be found at the junction of Boreenmanna Road and Rockboro Avenue on the south side of Cork City.
It's temporarily out-of-service, because there's a new house being built there, and the postbox is part of the wall. As you can see, however, the postbox has been preserved, and the new wall was effectively constructed and plastered around it.
I love the fact that, temporally speaking, the postbox is primary. So rather than a "wall embedded" box, this is really a postbox with a house built on to the back of it.