I am going to attempt to answer my own question. While the FGB is helpful, what a teacher said to me might close the loop. Irish generally doesn't like to end a sentence with a verb. So in that situation "ann" is often added to close the sentence and can be thought of in English as "in there," "there," or "exists" if needed. However, in these cases the "ann" is just a closer and is not supposed to detract from the subject. The Duolingo translation does all the above even though, to me, that translation does pull the word "it" out of thin air.
You weren't far off the first time.
If you want to say "she is a ..." you can say ... atá inti.
Déagóir atá inti - "she is a teenager"
Bean chliste atá inti - "she is a clever woman"
bean as mo thír féin atá inti - "she's a compatriot of mine"
inti is the feminine 3rd person singular prepositional pronoun for i, and the the masculine form is ann, so ceapaire atá ann could be "he's a sandwich" (if you were talking about your brother), but usually it means "it's a sandwich".
By looking at examples using inti it's easier to see which ann is in play here.
Tá ceapaire ann - "there's a sandwich there" (or the clunkier "a sandwich is there").
ceapaire atá ann - "it's a sandwich"
Compare it to Rúnaí atá ionat - "You are a secretary". ionat is the 2nd person singular prepositional pronoun for i, and ann is the masculine 3rd person singular.
Or think of it as, “A sandwich is in it , is it not?” The word “ann” really means “in it.” So the first portion of the sentence has connected two nouns (sandwich and it) without a copula. That’s worth a lot if you’ve struggled with copula sentences. And of course, that literal translation is an awkward English sentence so hence the translation in the example.