"We are girls."
Translation:Is cailíní muid.
Long time has passed since you asked, but just for other people's information: "Táimid cailíní" sound to my ears like "We be girls" sounds in English. I would understand what you mean, but it just is wrong somehow. Grammar is not my strong point, so not sure what the technical reason is.
"Sinn" should also be acceptable, even though the course uses only muid.