Some _______, or some wit, had dubbed him the King of Ireland, and he was playing the part rather well.

The sentence suggests that the blank should contain a noun, as it is used with the adjective 'some'. Given the context, the word should mean a fool, the opposite of 'wit'. As the sentence suggests the usage of 'or' in a mutually contradicting way, e.g. He was a genius or a fool. Let's look at the meanings of the given words: Ignoble- not honorable in character or purpose. Ignominious- deserving or causing public disgrace or shame. Ignoramus- an ignorant or stupid person. Boor- a rough and bad-mannered person. The only option that gives the intended meaning is Ignoramus. Therefore, the correct answer is option 3.