"The cat's man."
Translation:Fear an chait.
Fear an chait - "The man of the cat" - "The cat's man" (one man, one cat)
Fear na gcat - "The man of the cats" - "The cats' man" (one man, multiple cats)
Fir an chait - "The men of the cat" - "The cat's men" (multiple men, one cat)
Fir na gcat - "The men of the cats" - "The cats' men" (multiple men, multiple cats)
I'm pretty sure that's not what he's asking - he was trying to memorize the answers without necessarily understanding them, and he got his exercises mixed up.
cat isn't pluralized in Fear an chait - the an tells you that it's definitely not plural, for a start. An chait is the genitive singular, not the nominative plural. In the case of cat, the genitive singular and the nominative plural share the same form, but that isn't true for all words, and the grammar of Irish requires that second noun to be in the genitive, so it will automatically be interpreted as such.