Lugh's father is Cian of the Tuatha Dé Danann, and his mother is Ethniu, daughter of Balor, of the Fomorians. In Cath Maige Tuired their union is a dynastic marriage following an alliance between the Tuatha Dé and the Fomorians. In the Lebor Gabála Érenn Cian gives the boy to Tailtiu, queen of the Fir Bolg, in fosterage. In the Dindsenchas Lugh, the foster-son of Tailtiu, is described as the "son of the Dumb Champion".
A folktale told to John O'Donovan by Shane O'Dugan of Tory Island in 1835 recounts the birth of a grandson of Balor who grows up to kill his grandfather. The grandson is unnamed, his father is called Mac Cinnfhaelaidh and the manner of his killing of Balor is different, but it has been taken as a version of the birth of Lugh, and was adapted as such by Lady Gregory. In this tale, Balor hears a druid's prophecy that he will be killed by his own grandson. To prevent this he imprisons his only daughter in the Tór Mór (great tower) of Tory Island, cared for by twelve women, who are to prevent her ever meeting or even learning of the existence of men. On the mainland, Mac Cinnfhaelaidh owns a magic cow who gives such abundant milk that everyone, including Balor, wants to possess her. While the cow is in the care of Mac Cinnfhaelaidh's brother Mac Samthainn, Balor appears in the form of a little red-haired boy and tricks him into giving him the cow. Looking for revenge, Mac Cinnfhaelaidh calls on a leanan sídhe (fairy woman) called Biróg, who transports him by magic to the top of Balor's tower, where he seduces Eithne. In time she gives birth to triplets, which Balor gathers up in a sheet and sends to be drowned in a whirlpool. The messenger drowns two of the babies, but unwittingly drops one child into the harbour, where he is rescued by Biróg. She takes him to his father, who gives him to his brother, Gavida the smith, in fosterage.
There may be further triplism associated with his birth. His father in the folktale is one of a triad of brothers, Mac Cinnfhaelaidh, Gavida and Mac Samthainn, and his father in the medieval texts, Cian, is often mentioned together with his brothers Cú and Cethen. Lebor Gabála Érenn Two characters called Lugaid, a popular medieval Irish name thought to derive from Lugh, have three fathers: Lugaid Riab nDerg (Lugaid of the Red Stripes) was the son of the three Findemna or fair triplets, and Lugaid mac Con Roí was also known as mac Trí Con, "son of three hounds". In Ireland's other great "sequestered maiden" story, the tragedy of Deirdre, the king's intended is carried off by three brothers, who are hunters with hounds. The canine imagery continues with Cian's brother Cú ("hound"), another Lugaid, Lugaid Mac Con (son of a hound), and Lugh's son Cúchulainn ("Culann's Hound"). A fourth Lugaid was Lugaid Loígde, a legendary King of Tara and ancestor of (or inspiration for) Lugaid Mac Con.
As a young man Lugh travels to Tara to join the court of king Nuada of the Tuatha Dé Danann. The doorkeeper will not let him in unless he has a skill with which to serve the king. He offers his services as a wright, a smith, a champion, a swordsman, a harpist, a hero, a poet and historian, a sorcerer, and a craftsman, but each time is rejected as the Tuatha Dé Danann already have someone with that skill. But when Lugh asks if they have anyone with all those skills simultaneously, the doorkeeper has to admit defeat, and Lugh joins the court and is appointed Chief Ollam of Ireland. He wins a flagstone-throwing contest against Ogma, the champion, and entertains the court with his harp. The Tuatha Dé Danann are at that time oppressed by the Fomorians, and Lugh is amazed how meekly they accept this. Nuada wonders if this young man could lead them to freedom. Lugh is given command over the Tuatha Dé Danann, and he begins making preparations for war.
When the sons of Tuireann: Brian, Iuchar and Iucharba kill Lugh's father, Cian (who was in the form of a pig at the time), Lugh sets them a series of seemingly impossible quests as recompense. They achieve them all but are fatally wounded in completing the last one. Despite Tuireann's pleas, Lugh denies them the use of one of the items they have retrieved, a magic pigskin which heals all wounds. They die of their wounds and Tuireann dies of grief over their bodies.
Using the magic artifacts the sons of Tuireann have gathered, Lugh leads the Tuatha Dé Danann in the Second Battle of Mag Tuireadh against the Fomorians. Nuada is killed in the battle by Balor. Lugh faces Balor, who opens his terrible, poisonous eye that kills all it looks upon, but Lugh shoots a sling-stone that drives his eye out the back of his head, wreaking havoc on the Fomorian army behind. After the victory Lugh finds Bres, the half-Fomorian former king of the Tuatha Dé Danann, alone and unprotected on the battlefield, and Bres begs for his life. If he is spared, he promises, he will ensure that the cows of Ireland always give milk. The Tuatha Dé Danann refuse the offer. He then promises four harvests a year, but the Tuatha Dé Danann say one harvest a year suits them. But Lugh spares his life on the condition that he teach the Tuatha Dé Danann how and when to plough, sow and reap.
Lugh instituted an event similar to the Olympic games called the Assembly of Talti which finished on Lughnasadh (1 August) in memory of his foster-mother, Tailtiu, at the town that bears her name (now Teltown, County Meath). He likewise instituted Lughnasadh fairs in the areas of Carman and Naas in honour of Carman and Nás, the eponymous tutelary goddess of these two regions. Horse races and displays of martial arts were important activities at all three fairs. However, Lughnasadh itself is a celebration of Lugh's triumph over the spirits of the Otherworld who had tried to keep the harvest for themselves. It survived long into Christian times and is still celebrated under a variety of names. Lúnasa is now the Irish name for the month of August.
According to a poem of the dindsenchas, Lugh was responsible for the death of Bres. He made 300 wooden cows, and filled them with a bitter, poisonous red liquid which was then "milked" into pails and offered to Bres to drink. Bres, who was under an obligation not to refuse hospitality, drank it down without flinching, and it killed him.
Lugh is said to have invented the board game fidchell. He had a dog called Failinis.
He had several wives, including Buí and Nás, daughters of Ruadri, king of Britain. Buí lived and was buried at Knowth. Nás was buried at Naas, County Kildare, which is named after her. Lugh had a son, Ibic, by Nás. His daughter or sister was Ebliu, who married Fintan. One of his wives, Buach, had an affair with Cermait, son of the Dagda. Lugh killed him in revenge, but Cermait's sons, Mac Cuill, Mac Cecht and Mac Gréine, killed Lugh in return, drowning him in Loch Lugborta. He had ruled for forty years. Cermait was later revived by his father the Dagda, who used the smooth or healing end of his staff to bring Cermait back to life.In the Ulster Cycle he fathered Cúchulainn with the mortal maiden Deichtine. When Cúchulainn lay wounded after a gruelling series of combats during the Táin Bó Cuailnge (Cattle Raid of Cooley), Lugh appeared and healed his wounds over a period of three days.
In Baile in Scáil (The Phantom's Trance), a story of the Historical Cycle, Lugh appeared in a vision to Conn of the Hundred Battles. Enthroned on a daïs, he directed a beautiful woman called the Sovereignty of Ireland to serve Conn a portion of meat and a cup of red ale, ritually confirming his right to rule and the dynasty that would follow him.
In the Fenian Cycle the dwarf harper Cnú Deireóil claimed to be Lugh's son.
The Luigne, a people who inhabited Counties Meath and Sligo, claimed descent from him.
Ainle is listed as the son of Lug Longhand and is killed by Curnan the Blacklegged in the Rennes Dinsenchas. Ainle, whose name means "champion" is described as being renowned and glorious, but in the same poetic verse is also described as being a weakling with no grip in battle.
In the Dindsenchas, Luat the son of Scal Balb (another name of Cian) is mentioned as the husband of Bairend.
Lug possessed a number of magical items, retrieved by the sons of Tuirill Piccreo in Middle Irish redactions of the Lebor Gabála. Not all the items are listed here. The late narrative Fate of the Children of Tuireann not only gives a list of items gathered for Lugh, but also endows him with such gifts from the sea god Manannán as the sword Fragarach, the horse Enbarr (Aonbarr), the boat Scuabtuinne / Sguaba Tuinne ("Wave-Sweeper"), his armour and helmet.
The lore around Lug's Spear is traced as follows:
Lugh's spear (sleg), according to the text of The Four Jewels of the Tuatha Dé Danann, was said to be impossible to overcome, taken to Ireland from Gorias (or Findias).
Lugh obtained the Spear of Assal (Irish: Gae Assail) as fine (éric) imposed on the children of Tuirill Piccreo (or Biccreo), according to the short account in Lebor Gabála Érenn (Poem LXV, 319), which adds that the incantation "Ibar (Yew)" made the cast always hit its mark, and "Athibar (Re-Yew)" caused the spear to return.
In a full narrative version called [A]oidhe Chloinne Tuireann (The Fate of the Children of Tuireann), from copies no earlier than the 18th century, Lugh demands the spear named Ar-éadbair or Areadbhair (Early Modern Irish: Aꞃéadḃaiꞃ) which belonged to Pisear, king of Persia. Its tip had to be kept immersed in a pot of water to keep it from igniting, a property similar to the Lúin of Celtchar. This spear is also called "Slaughterer" in translation.
There is yet another name that Lugh's spear goes by: "A [yew] tree, the finest of the wood" (Early Modern Irish: eó bo háille d'ḟíoḋḃaiḃ), occurring in an inserted verse within The Fate of the Children of Tuireann. "The famous yew of the wood" (ibar alai fhidbaidha) is also the name that Lugh's spear is given in a tract which alleges that it, the Lúin of Celtchar and the spear Crimall that blinded Cormac Mac Airt were one and the same weapon (tract in TCD MS 1336 (olim H 3. 17), col. 723, discussed in the Lúin page).
Lugh used the "sling-stone" (cloich tabaill) to slay his grandfather, Balor the Strong-Smiter in the Battle of Magh Tuired according to the brief accounts in the Lebor Gabála Érenn. The narrative Cath Maige Tured, preserved in a unique 16th century copy, words it slightly different saying that Lugh used the sling-stone (here liic talma § 133, i.e. lía "stone" of the 'tailm "sling") to destroy the evil eye of Balor of the Piercing Eye (Bolur Birugderc).
A certain poem recorded by O'Curry in English translation says that the missile fired by Lugh was a tathlum (táthluib "(slingstone made of) cement").
Lugh's projectile weapon, whether a dart or missile, was envisioned by symbolic of lightning-weapon T. F. O'Rahilly. Lugh's sling rod, named "Lugh's Chain", was the rainbow and the Milky Way. Unlike the rod-sling, Lugh had no need to wield the spear himself. It was alive and thirsted so for blood that only by steeping its head in a sleeping-draught of pounded fresh poppy seeds could it be kept at rest. When battle was near, it was drawn out; then it roared and struggled against its thongs, fire flashed from it, and it tore through the ranks of the enemy once slipped from the leash, never tired of slaying.
Lugh is also seen girt with the Freagarthach (better known as Fragarach), the sword of Manannán, in the assembly of the Tuatha Dé Danann in the Fate of the Children of Tuireann.
Lugh had a horse named Aenbharr which could fare over both land and sea. Like much of his equipment, it was furnished to him by the sea god Manannán mac Lir. When the Children of Tuireann asked to borrow this horse, Lugh begrudged them, saying it would not be proper to make a loan of a loan. Consequently, Lugh was unable to refuse their request to use Lugh's currach (coracle) or boat, the "Wave-Sweeper" (Irish: Sguaba Tuinne).
In the Lebor Gabála, Gainne and Rea were the names of the pair of horses belonging to the king of the isle of Sicily [on the (Tyrrhene sea)], which Lug demanded as éric from the sons of Tuirill Briccreo.
Failinis was the name of the whelp of the King of Ioruaidhe that Lugh demanded as éiric (a forfeit) in the Oidhead Chloinne Tuireann. This concurs with the name of the hound mentioned in an "Ossianic Ballad", sometimes referred to by its opening line "Dám Thrír Táncatair Ille (They came here as a band of three)". In the ballad the hound is called Ṡalinnis (Shalinnis) or Failinis (in the Lismore text), and belonged to a threesome from Iruaide whom the Fianna encounter. It is described as "the ancient grayhound... that had been with Lugh of the Mantles, / Given him by the sons of Tuireann Bicreann;..."
O'Curry's excerpt ends here, but the subsequent verse runs "The three full-fledged heroes are called Sél, Donait and Domhnán. The dog of the fairest figure, Failinis was brought to Finn". These threesome also appear in Acallamh na Sénorach though in that work the wonder-dog is called Fer Mac.
Lugh's name has been interpreted as deriving from the Proto-Indo-European root *leuk-, "flashing light", and he is often surrounded by solar imagery, so from Victorian times he has often been considered a sun god, similar to the Greco-Roman Apollo though historically he is only ever equated with Mercury. He appears in folklore as a trickster, and in County Mayo thunderstorms were referred to as battles between Lugh and Balor, so he is sometimes considered a storm god: Alexei Kondratiev notes his epithet lonnbeimnech ("fierce striker") and concludes that "if his name has any relation to 'light' it more properly means 'lightning-flash' (as in Breton luc'h and Cornish lughes)". However, Breton and Cornish are Brythonic languages in which Proto-Celtic *k did undergo systematic sound changes into -gh- and -ch-.
Words containing Lu, as in the word Lugh itself, or lo or le, have appeared for millennia always meaning light or sun or sun god. Luwian Apaliunas, Hurrian Aplu, Etruscan Apulu, Homeric Greek: Ἀπόλλων, that is λω, Latin Apollo. The form Apaliunas (]x-ap-pa-li-u-na-aš) is attested as a god of Wilusa in a treaty between Alaksandu of Wilusa interpreted as "Alexander of Ilios", and the Hittite great king Muwatalli II ca. 1280 BC.
Luwian is closely related to Hittite, and was among the languages spoken during the second and first millennia BC by population groups in central Anatolia, Anatolia (from Greek Aνατολή Anatolē—"East"; also Asia Minor. When the Illyrians migrated to Italy and founded Luceria in Apulia, a temple to Minerva was built. Minerva is the Etruscan and Roman equivalent of Athena. The arms (armament and weapons) of Diomedes, given to him by Athena in the Trojan War, were said to be were preserved in her temple.
The Lusitanians (or Lusitani in Latin) were an Indo-European people living in the Western Iberian Peninsula. Endovelicus was the most important god. António da Visitação Freire classified the name of "Endovelicus" as a mixed Celtic and Phoenician name, adapted to the Roman language. The "end-" radical would be from Celtic languages, "bel" (or "vel-") would be Phoenician for "lord", and "-cus" is a usual word termination in Latin. The name would suggest Bal, Bel, or Vel, the god Belenus (also Belenos) was a deity worshipped in Gaul, Britain, and Celtic areas of Austria and Spain. In the Roman period Belenus was identified with Apollo. Belisama has been claimed to be the consort of Belenus and she was identified with Minerva/Athena. It would seem that the word Lugh is related to every Indo-European language word meaning light.
Lugh's mastery of all arts has led many to link him with the unnamed Gaulish god Julius Caesar identifies with Mercury, whom he describes as the "inventor of all the arts". Caesar describes the Gaulish Mercury as the most revered deity in Gaul, overseeing journeys and business transactions. Juliette Wood interprets Lugh's name as deriving from the Celtic root *lugios, "oath", and the Irish word lugh connotes ideas of "blasphemy, cussing, lies, bond, joint, binding oath", which strengthens the identification with Mercury, who was, among other attributes, a god of contracts.
It is also worth noting that parallels exist between the Irish Lugh, Gaulish Lugus, German Wotan, the English Woden, and Norse Odin. Odin was worshipped by the Norse as a god of war among other things, including poetry and the arts. Odin may have replaced Tyr as god of war among north Germanic peoples. As such, it may be that Lugh was also worshipped as a god of war by the Irish. On that note it is worth noting that the ultimate Irish warrior hero Cu Chulainn is cited as the son of Lugh.