"I am a girl."
Translation:Is cailín mé.
This page is a really clear explanation of the difference between "tá" and "is" (assuming you were confused by this like I was!) Basically "tá" is never the copula - so if you're describing something with an adjective, noun, or pronoun, present tense is always "is", just like in English. http://nualeargais.ie/gnag/kopul4.htm