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
In Ireland, where people know what a sliotar is, it's usually called a sliotar in English as well. For most of the people doing this course, direct experience of hurling is not frequent, so the course uses the basic descriptive terms, as the word "sliotar" doesn't exist in the English vocabulary of those learners.
It seems like a reasonable approach - even O'Neills, who make an awful lot of stuff for the GAA, refer to "hurling balls".
Twenty thousand Irish words I want to learn and and sliotar is dead last. The sound of it is like an old Barry Manilow song; you just can't get rid of it. How do you say "Mandy" in Irish?