He couldnt find Ireland on a map of Ireland

exp.

Heard in Tipperary

Describing someone who is very stupid.

In use

Tom: "Look at that eejit over there!" John: "Yeah, he's so thick. He couldnt find Ireland on a map of Ireland"

Posh English

Tom: Observe that fool over there! John: Indeed, he is so dense he could not locate Ireland upon a map of Ireland.