"What is your name, boy?"
Translation:Dè an t-ainm a th' ort, a bhalaich?
Balach becomes "a bhalaich" because it's in the vocative case, which you use when you're speaking to someone. Look at the tips and notes for the lesson; you can't see them from the app version, but if you open duolingo.com in any browser including your phone browser and log in you can find them from there. Go to the lesson and open it and you'll see "tips and notes"