If you want to say they eat, you need the word they--siad.
When the subject is mé or muid, you can use the synthetic forms--the ones where the subject and verb are smooshed together, like ithim and ithimid. Do not add mé or muid to these forms, unless you want to say I I eat.
You do need to add a subject when the verb is itheann because that's the form to use with tú, sé, sí, muid (if you don't use ithimid), sibh, and siad. Itheann siad = They eat Itheann siad na páistí = They eat the children. Since Irish is a VSO (verb-subject-object) language, you will usually find the subject right after the verb and the object after that.
Irish doesn't make as big a deal about subjects and objects as, say, German. The nouns are the same for subjects and objects, but the pronouns change: sé, sí, siad (subjects) become é, í, iad (objects). Many people use tú for the subject and thú for the object, but I don't know if that is required.
Itheann tú iad. = You eat them. Itheann siad thú. = They eat you.
Thank you so much! I think I was wrapping my mind around the synthetic form, and thinking that all conjugations had a synthetic form. Like in Spanish where one can drop the subject because the conjugation of the verb generally indicates the subject. I would hate to accidentally cannibalize anyone!
Quoting the very first lines of the Tips & Notes for the very first skill, Basics 1
Welcome to Duolingo's Irish course! In this course you will learn the official standard (an Caighdeán Oifigiúil) of Irish. But note, this is a written, and not a spoken standard. Irish is spoken in three main dialects, corresponding to three Irish provinces of Munster (south), Ulster (north), and Connacht (west). The audio in this course was recorded by a native speaker of the Connacht dialect.