Brain balls

In Geoffrey Keating's 17th century Foras Feasa ar Éirinn (or 'History of Ireland') in 'An Account of the Death of Connor, King of Ulster' he records the background of the tradition, and a Christianised tale relating to it.

Later the provinces of Connacht and Ulster became at war, and Cet used the ball as a sling stone, wounding Connor grievously in the head - this began the fulfillment of a prophecy that Mesgegra would be revenged on Ulster after his death - King Connor's doctors advised that extreme stress of exertion might open the ball's wound and kill him, and so he lived carefully for seven years.

[2] In Irish dolmens balls have been fairly common finds, often found in the structure's floor or lower level detritus, and often associated with bones - materials identified as forming the balls include marble (thought to be a slingshot); ironstone ; calcium carbonate; porphyry; and syenite - with sizes recorded were from approximately 0.75 to 3 inches (19 to 76 mm).

Wakemen submitted a sample for analysis - the ball was approximately 1.5 inches (38 mm) diameter, light yellowish, and friable, containing some harder material.

An example is the tathlum made by the Tuatha Dé Danann from the "blood of toads, bears, and vipers, mixed up with sea-sand and hardened" - this ball was used by Lugh to kill Balor.