From
A Smaller Social History of Ireland by Patrick Weston Joyce:
"Sling and Sling-stones.—A much more effective instrument for stone-throwing was the sling, which is constantly mentioned in the Tales of the Táin, as well as in Cormac's Glossary and other authorities, in such a way as to show that it formed an important item in the offensive arms of a warrior. The accounts, in the old writings, of the dexterity and fatal precision with which Cuculainn and other heroes flung their sling-stones, remind us of the Scriptural record of the 700 chosen warriors of Gibeah who could fight with left and right hand alike, and who flung their sling-stones with such aim that they could hit even a hair, and not miss by the stone's going on either side (Judges xx. 16).
The Irish used two kinds of sling. One, which was called by two names teilm and taball [tellim taval] consisted of two thongs attached to a piece of leather at bottom to hold the stone or other missile: a form of sling which was common all over the world, and which continues to be used by boys to this day. The other was called crann-tabaill, i.e. 'wood-sling' or 'staff-sling,' from crann, 'a tree, a staff, a piece of wood of any kind'; which indicates that the sling so designated was formed of a long staff of wood with one or two thongs—like the slings we read of as used by many other ancient nations. David killed Goliath with a staff-sling. Those who carried a sling kept a supply of round stones, sometimes artificially formed. Numerous sling-stones have been found from time to time—many perfectly round—in raths and crannoges, some the size of a small plum, some as large as an orange, of which many specimens are preserved in museums."