"It is an orange."
Translation:Is oráiste é.
The simplest way to look at it is the cupola (is - é here) is mostly used to describe equivalence or state a fact about something's nature, rather than something that thing does. so 'Ithim oráiste' because the orange is something I eat, but 'is oráiste é' because that is what the orange is. That's a really bad explanation and the cupola is more complex than that but that's the basic idea.