Alas, I see in translations that the meaning is very contested
http://thetaintorque.com/ChariotTranslation.html

return stroke
ricochet shot
stunning shot

are the three translations.

Fenian Cycle-Battle of Ventry (Cath Finntraga)

Two manuscripts survive dating to the 15th century. The earliest texts it is suggested dated to the 12 th century. However it is known that just as in all the other pre-Christian tribes of the British Isles, the stories/fables date back many years prior, being passed by word of mouth across the generations.

The tales of Finn like all the mythical fables are not recorded history. They are symbolic stories originally telling of the spirituality of the tribe.

Two other important facts must also be kept in mind when trying to interpret the texts:

a) those that survive where written a thousand years after Christianity infected the native people's. The last point in history where the stories remained true to their pagan spirituality.
b) the Christian authors radically changed the original stories as can be seen by the many similarities with Greek texts and the near elimination of all pagan spirits. 

It should be remembered that the pope ordered that pagan temples should not be destroyed, but that the iconography be replaced with compatible Christian symbols. As such temples to a / the goddess figure morphed into churches of St Mary (the mother of God), temples to a God morphing into churches of Jesus or St. Peter for example. For proof look no further than  The Sisters of Brigid in Killdare ( meaning Church of the Oak)

For those not versed in the ancient ways, Brigid was the name of the Mother Goddess. The mother of the male fertility God figure who dies and is reborn each year. His symbol is the sun and fire is his element.

Christianity stole and then morphed the pagan beliefs to match their own. In this way Rome achieved through the piety of the pagan people, what she had failed to do at the point of a sword.

So, reading these texts as a way to learn the facts of pre-Christian Ireland, is no different to reading the bible as literal truth.

Ran across a sling reference in the Annals of the Four Masters. It talks about about the death of King Crimhthainn, shortly after an expedition he had been on. He brought back wonderful items, including " A conquering sword, with many serpents, of refined messy gold on it, a shield with with bosses of bright silver, a spear, from which the wound inflicted by no one recovered, a sling, from which no erring shot was ever discharged; and 2 greyhounds, with a silver chain between them."  M9.1

William Sayers, "Martial Feats in the Old Irish Ulster Cycle"
The Canadian Journal of Irish Studies 9 (1983), pp. 45-80.

Discusses martial feats, ("Cles") in Old Irish tales: heroes frequently perform tricks or feats involving arms, as a demonstration of prowess. Some of the tales give lists of such feats. Cu Chulainn is said to have performed them in sequence every morning, "swift as a cat making for cream".

Tales compiled in C12th, oral tradition written down in C7th, might reflect culture of C4th, events placed a few centuries earlier (i.e. Christian-era interest in tales dating to pre-Christian era and themselves placed even earlier).

There are jugging or dexterity tricks / feats with "apples" (some kind of thrown spherical weapon), sword, shield and spear; leaping feats (like the famous "salmon jump"); weapons throwing; various types of blows; the taithbeim, "return stroke" which describes both the stunning of birds with the slingstone, and a type of sword stroke (not clear what's involved: possibly describing the path of the sword, a "short-arm" blow ?), and finally the "thunder-trick", torannchless, by which the "thunder" of the slingstone lays low one (or many) men, not clear if it's the "sound' of the sling or the actual strike of the stone. 

So less slinging than expected, but a bit; no clarity on the mysterious "return-stroke" of the sling (the expression is also used for thrown swords).

In "The Violent Death of Medb" (published by V. Hull, Speculum, 1938), a tale now preserved in a C12th compilation,    Queen Medb, while bathing, is seen by Furbaide, the son of Medb's sister Clothru, overthrown and killed by Medb. Furbaide was eating a piece of cheese; he did not tarry to seek a stone, but loaded the cheese in his sling, let fly, and struck Medb on the crown of the head, killing her.

Must have been some very hard cheese indeed.