"They eat the cake."
Translation:Itheann siad an cáca.
I don't know if it's still spoken, but scone bread used to be called "cake" (cáca). That would mean one would have to differenciate between bread and cake, so "milis" (meaning sweet) would have been added to cake, hence "sweet cake" (cáca milis). In Danish, "melis" is a type of sugar, and the origin is probably from medieval latin "saccharum melitense" (sugar from Malta).
It should be itheann siad an cáca. They won't count an answer wrong due to fada, but the sentence structure needs to be VSO. Verb: Itheann/eat. Subject: Siad/they. Object: an cáca or an císte/the cake. Milis means sweet, but isn't fully necessary when speaking about cake. There are other types of cake, however, and this will help describe a type of cake.