why is thusa used here instead of thu? Is it plural? Or something else?
No. It's emphatic. Like using an exclamation mark in English. The plural would be sibhse. (This is all explained in the course notes by the way which are viewable on Duolingo's website.)
ah, thanks. I always forget about course notes...
Sound quality is bad.
Is this the usual way to ask someone who they are? I ask because where I am from, saying "who are you?" would generally be perceived as rude. But I know each culture has their own customs and what is and is not considered polite.