Conall Cernach is a hero of the Ulaid in the Ulster Cycle of Irish mythology. His epithet is normally translated as "victorious" or "triumphant", although it is an obscure word, and some texts struggle to explain it. Alternative meanings include "angular, having corners" or "swollen". He takes a secondary role in the cycle to his cousin and foster-brother Cú Chulainn, but becomes the Ulaid's most prominent hero after Cú Chulainn's death. He has a bitter rivalry with Cet mac Mágach, a warrior of the Connachta and Conall's uncle. Unusually for a character from the Ulster Cycle, several medieval Irish dynasties counted Conall as an ancestor.
Conall's father is Amergin the poet, and his mother is Findchóem, who in most texts is the sister of the Ulaid king, Conchobar mac Nessa, but in some is the sister of the Connacht warrior Cet mac Mágach. Amergin and Findchóem's marriage is barren, until Findchóem visits a druid who advises her to drink from a certain well. She takes a drink from the water, swallows a worm with it, and becomes pregnant. Cet vows to protect his sister until she gives birth. When her son is born, a druid prophesies that he will kill more than half of the Connachta, and will always have a Connachtman's head on his belt. Cet takes the child, puts him under his heel and tries to break his neck. He fails, but leaves Conall with a permanently crooked neck.
Mac Da Thó's Pig
Conall has a fierce rivalry with Cet for the rest of his life. He shames Cet at a feast at the house of Mac Dá Thó, a hospitaller of Leinster, when the warriors of Connacht and Ulster compete for the champion's portion by boasting of their deeds. Cet reminds all comers how he has bested them in combat, including emasculating Celtchar with his spear, and is acknowledged as the greatest hero present. However, just as Cet is about to carve, Conall arrives, and his boasts top even Cet's. Cet admits defeat, but claims that if his brother Anlúan were present, his feats would top even Conall's. Conall responds by tossing him Anlúan's freshly severed head.
He also competes for the champion's portion at a feast held by the troublemaker Bricriu mac Carbada, albeit with less success. Before the feast Bricriu goes in turn to Conall, Lóegaire Búadach and Cú Chulainn, and promises each of them the champion's portion. When the feast starts each of the three warriors' charioteers stands up and claims the champion's portion for his master. A fight breaks out between Conall, Láegare and Cú Chulainn, until Conchobar, Fergus and Sencha intervene to separate them. Meanwhile, Bricriu goes to each of the three heroes' wives - Conall's wife Lendabair, Lóegaire's wife Fedelm, and Cú Chulainn's wife Emer - and promises them precedence at the feast, and when the women approach, Conall, Lóegaire and Cúchulainn are almost set to violence again. Emer is the first to enter, as Cú Chulainn lifts the side of the house up to let her in, tipping Bricriu into a ditch. The Ulstermen ask first Ailill and Medb, king and queen of Connacht, then Cú Ruí mac Dáiri, king of Munster, to adjudicate the dispute. In every test set, Cú Chulainn comes out on top, but neither Conall nor Lóegaire accept the result. Finally, a hideous, giant churl, carrying a huge axe, appears at Emain Macha. He challenges each of the three heroes to cut off his head, and then allow him to return the next day to cut off the hero's head. Lóegaire accepts the challenge and cuts off the churl's head, and the churl picks up his head and leaves. He returns the next day, but Lóegaire is nowhere to be seen. Conall is the next to take up the challenge, but he too does not fulfil his side of the bargain. Finally Cú Chulainn cuts off the churl's head, and submits himself to the churl's axe the following day as promised. The churl spars him, reveals himself as Cú Ruí, and declares that Cúchulainn should have the champion's portion undisputed at any feast held by the Ulaid
The Cattle Raid of Fráech
In the saga Táin Bó Fraích ("The Cattle Raid of Fráech"), Conall helps the Connacht hero Fráech recover his abducted wife and sons and stolen cattle. They track them to Scotland, southwards through Britain, across the English Channel, through Lombardy, to the Alps, where they meet an Irish girl herding sheep. She tells them the land is ruled by warriors who steal cattle from far and near, and had recently brought back Fráech's cattle and family. She advises them to go to the woman who tends the cows, who warns them that the fort where Fráech's wife is kept is guarded by a serpent, but promises to leave the gate open for them. When they attack the fort, the serpent leaps into Conall's belt, and does him no harm. They liberate Fráech's family, take all the cattle and treasure, and go back to Ireland the way they came.
The Battle of Howth
After the Ulster poet Athirne provokes a war with Leinster, Conall fights the Leinster king Mes Gedra in single combat. Mes Gedra has lost a hand in an earlier fight, so Conall fights him with one hand tucked into his belt. He wins, taking his opponent's head as a trophy. When he puts Mes Gedra's head on his shoulder, it straightens his neck. Conall's charioteer can't carry the head, so he cuts out the brain and preserves it by mixing it with lime. The calcified brain is later stolen by Cet and used to kill Conchobar.
The death of Cú Chulainn
Conall and Cú Chulainn swore to each other that whoever was killed first, the other would avenge him before nightfall. When Lugaid, son of Cú Ruí, and Erc, son of Caipre Nia Fer, kill Cúchulainn, Conall pursues them. Lugaid has also lost a hand, and Conall again fights one-handed, but this time he only wins after his horse takes a bite out of Lugaid's side. He takes both their heads, and when he takes Erc's head back to Tara his sister, Achall, dies of grief.
Final showdown with Cet
Conall pursues Cet after he has made a raid on Ulster, killing twenty-seven men and taking their heads. It has snowed, so he is able to follow his trail. He catches up with him, but is reluctant to face him until his charioteer chides him for cowardice. They meet at a ford, and Conall kills Cet in a ferocious combat that leaves Conall near to death himself. He is found by Bélchú of Breifne, a Connachtman, who takes him home, tends his wounds, and plans to fight him when he is fit. But Bélchú soon regrets his honourable behaviour and asks his three sons to kill Conall as he lies in his sickbed. Conall overhears and forces Bélchú to take his place in the bed, and when his sons arrive they killed their father instead. Conall then kills the three of them and takes all four heads home.
After Conchobor and his son, Cormac Cond Longas, have been killed, Conall is offered the kingship of the Ulaid, but he refuses, putting forward instead his foster-son, Conchobar's younger son Cúscraid, who is proclaimed king. In his declining years he goes to live with Ailill and Medb of Connacht, who are best placed to look after him, since they have the resources to satisfy his enormous appetite. Ailill is seeing another woman behind Medb's back, so Medb incites Conall to kill Ailill, something he is happy to do as Ailill had killed Fergus mac Róich. He does the deed and flees, but the men of Connacht pursue and kill him at a ford. Oral tradition places this at the town of Ballyconnell, County Cavan.