A sliotar ( or sliothar ) - is a hard solid sphere slightly larger than a tennis ball , consisting of a cork core covered by two pieces of leather stitched together . A sliotar or sliothar ( ; may derive from Irish sliabh , meaning `` mountain '' and thar meaning `` across '' ) is a hard solid sphere slightly larger than a tennis ball , consisting of a cork core covered by two pieces of leather stitched together .