"She is a girl."
Translation:Is cailín í.
From my understanding, there is a difference between "state" and "essence" forms of "to be". An "essence" never changes, as in "is fear mé" because (for myself anyways) I will always be male. However, "state" can change as in "tá mé fuar" ("I am cold") since I might warm up later.
(As an aside, note the placement of "fuar" in the last sentence. We haven't quite gotten to that yet, but I double-checked with both Google Translate and a few online references so I believe it to be correct)
It's not an accurate answer.
1) There is no tá in Itheann sí úll - "She eats an apple".
2) tá sé marbh - "he is dead", which is usually a pretty permanent state.
You use sé, sí and siad for the subject of a verb (other than the copula is) when they occur adjacent to the verb. You use é, í and iad for the object of a verb (itheann sí é - "she eats it", cloiseann sé iad - "he hears them"), and with the copula - Is cailiín í.
You can read more detail about the structure of sentences that use the copula is in the stickied post Eureka Moments with the Irish Copula: A Crib Sheet.