"An feirmeoir í? Ní hea."
Translation:Is she a farmer? She is not.
Pronouns following ní (the negative form of is) that begin with a vowel undergo H-prothesis.
The appropriate pronoun in the response is indeed ea because the question asked is one of classification. If it were a question of identification, e.g. An ise an feirmeoir? (“Is she the farmer?”), then the response would use í (either Is í [“She is”] or Ní hí [“She isn’t”]) rather than ea.