"Is he a doctor?"
Translation:An dochtúir é?
An é an dochtúir? or An eisean an dochtúir?
(Gramadach na Gaeilge suggests that the emphatic eisean should be used rather than the plain é, but that really depends on the question you are asking. If you are asking which of the different people in front of you is the doctor, eisean makes more sense, if you're trying to find out what the profession of the person you are taking about is, é is better).