Masculine definite nouns get lenited in the genitive singular and eclipsed in the genitive plural:
- an fear (“the man”)
- an fhir (“of the man”)
- na fir (“the men”)
- na bhfear (“of the men”)
When the lenition and eclipsis are removed, the nominative singular form matches the genitive plural form, and the nominative plural form matches the genitive singular form.
Sorry for the confusion - this sentence looks like it has been deleted from our side, but it is still showing up in the live course. We call these "ghost sentences". It has been reported to the Duolingo staff who will hopefully be able to remove it completely. Genitive sentences should not be appearing earlier than the genitive skill.