It's the ball used in hurling. It's the same size as a tennis ball but harder, with a distinctive curved ridge. It's usually called a sliotar in English as well
Is it a regional or a Duolingo thing that it's "hurling" rather than "hurley"? Yet https://www.teanglann.ie/en/eid/hurley allows "hurley" and "hurley-stick". As a kid in the north it was always hurley.
It's not a Duolingo thing: