"Is he poor or rich?"
Translation:An bhfuil sé bocht nó saibhir?
saibhir and daibhir are usually used as adjectives (An bhfuil sé daibhir nó saibhir), not nouns, but they can be used as nouns (though "poor" and "rich" are adjectives in the source sentence in this exercise).
But I think the copular form would be An daibhir nó saibhir é though it means "Is he a poor man or a rich man?* not "Is he poor or rich?".
It's definitely the interrogative form of the copula that is involved, and what you say makes sense. I guess I was thinking of constructions such as 'is deas é an duine' or 'ní furasta í an teanga seo' (http://www.nualeargais.ie/gnag/gram.htm under 'The copula and adjectives - with the predicative adjective'). Perhaps what I should have said was 'An bocht nó saibhir é?'