^(Please note I've moved my account to /u/Bealoideas, so if you have any further questions on this topic, I would be happy to answer messages sent there.)

So I have a bit more time now, so I'm going to flesh out this response a bit for people who may not have a lot of knowledge about the subject, so they can better understand what Gargatua is inquiring about. Cú Chulainn is a mythic hero in the Ulster Cycle of Ireland, which is a medieval collection of literature depicting a pre-Christian Ireland. Cú Chulainn (his birth name is Sétanta, Cú Chulainn means literally 'The Hound of Culann' after he killed Culann's watchdog and swore to take its place) is a virtually unmatched warrior who fights for the Ulster region of Ireland. Cu Chulainn appears most famously in the Táin Bó Cúailnge, or the Cattle-Raid of Cooley, where is engages in sole combat against a massive army commanded by Queen Medb of Connacht.

Cú Chulainn is notable for his great youth, with many of his exploits occurring when he is only a few years old, for his devastating martial skill, and as the question notes, a special weapon the Gáe Bolg (or Ga Bolga in modern Irish), which is pointed and barbed so fiercely that wielding it requires special training that only Cú Chulainn possesses, and whose wounds are utterly fatal.

With that said, and though I am not by any means a military historian, the answer unfortunately is basically no: Cú Chulainn was adept with the javelin, and certainly this weapon was used in Ireland, but the barbed javelin, the ga bolga, is likely as fanciful and hyperbolic as Cú Chulainn himself - I believe it has to be cut out of a person after impaling them, so filled they are with barbs. Indeed he has several exotic weapons, thinking of the Táin Bó Cúailnge particularly, his carbad searrdha or Scythed Chariot is a prominent example. A sort of Gladiator style bladed affair for cutting down opponents. War Chariots in general were probably not used in Ireland, despite their appearances in the literary tradition, though they did appear on the continent.

And to answer the second part of your question: Essentially the ga bolga was meant to be Cú Chulainn's trump card, when he was otherwise evenly matched, which was rare. In the Táin Bó Cúailnge he uses it to kill Ferdia, his sort-of foster-brother, both of them having been trained by Scáthach, a sort of Scottish woman warmaster. Since I have Kinsella's translation of the text beside me, here is the relevant passage:

Cúchulainn caught [the gae bolga] in the fork of his foot and sent it casting toward Ferdia and it went through the deep and sturdy apron of twice-smelted iron, and shattered in three parts the stout strong stone of the size of a mill-stone, and went coursing through the highways and byways of his body so that every single joint filled with barbs.

The Táin Trans. Thomas Kinsella. Oxford University Press 1969. p. 196.

This follows a very lengthy set of engagements, both martial and verbal between them, and Cúchulainn kills Ferdia very reluctantly. I believe he also kills his son with the same weapon, so there is probably a theme there as a weapon of pyrrhic victory.

Sources just off the top of my head are Daithi O hÓgain's The Lore of Ireland for general information on Cú Chulainn and Barry Cunliffe's The Ancient Celts for background on the Continental Celtic tribes of Europe.

Since there is quite a lot of discussion below on comics in the genre of Irish material, I'll promote the lovely series of graphic novels of Irish myths and legends written in Irish, of which The Táin is my favorite. There are translations online if you're not a gaeilgeoir.

Appendix

Irish orthography, particularly dealing with early Irish sources, is a bit...varied. So you may see terms I've used here spelled differently elsewhere. My own usage is just my preference, such as I mostly prefer Old Irish terminology, ignore Middle Irish (because it's overwrought), and mix in modern Irish because a lot of secondary sources use that, and it gets muddled. In Old Irish there's really only two vowel types, so Gáe Bulg and Gáe Bolg are identical, with Ga Bolga being the modern Irish, which I used because Daithi O hÓgain was using it when I sourced him. Cú Chulainn can be rendered about twenty different ways, the one I've used is my preference, Kinsella uses another without spacing in my excerpt. Ferdia is modern Irish as well, because O hÓgain used it, in the original material Ferdiad would be the appropriate form. Irish is a beautiful language, and I encourage everyone who has any interest to learn some.

This should be cross-posted to /r/ireland as well. May get some info there. Slainte.