"Do you want more?"
Translation:An bhfuil níos mó uait?
tuilleadh definitely should be accepted, but I'm not sure about the ar mhaith leat. While it has the same meaning, Duolingo's notorious about wanting the word-for-word translation (with ar mhaith leat being "would you like").
Though, I agree - ar mhaith leat tuilleadh sounds more native (heck, just using tuilleadh instead of níos mó)