Conall Cernach (Conall the Victorious) is a heroic warrior of the Ulaid in the Ulster Cycle of Irish mythology. His father was Amairgin mac Echit and his mother was Findchoem . He has a fierce rivalry with the Connacht warrior Cet mac Mágach, and is said to have always slept with a the head of a Connachtman under his knee. He had a crooked neck.

He shamed Cet at a feast at the house of Mac Dá Thó, a hospitaller of Leinster, when the warriors of Connacht and Ulster competed for the champion's portion by boasting of their deeds. Cet reminded all comers how he had bested them in combat, including emasculating Celtchar with his spear. However, just as Cet was about to carve, Conall arrived, and his boasts topped even Cet's. Cet admitted defeat, but claimed that if his brother Anlúan were present, his feats would top even Conall's. Conall responded by tossing him Anlúan's freshly severed head.

He also competed for the champion's portion at Briccriu's feast, with Cúchulainn and Lóegaire Búadach, but this time Cúchulainn came out on top.

He helped the Connacht hero Fráech recover his wife and cattle after they had been stolen and taken to the Alps.

He fought Mes Gedra, king of Leinster, in single combat following a battle provoked by the Ulster poet Athirne. Mes Gedra had lost a hand in an earlier fight, so Conall fought him with one hand tucked into his belt. He won, taking his opponent's head as a trophy. When he put Mes Gedra's head on his shoulder, it straightened his neck. Conall's charioteer couldn't carry the head, so he cut out the brain and preserved it by mixing it with lime. The calcified brain was later stolen by Cet and used to kill Conchobar mac Nessa.

Conall and Cúchulainn had sworn to each other that whoever was killed first, the other would avenge him before nightfall. When Lugaid mac Con Roí and Erc son of Cairbre Nia Fer killed Cúchulainn, Conall pursued them. Lugaid had also lost a hand, and Conall again fought one-handed, but this time he only won after his horse took a bite out of Lugaid's side. He took both their heads, and when he took Erc's head back to Tara his sister, Achall, died of grief.

Conall pursued Cet after he had made a raid on Ulster, killing twenty-seven men and taking their heads. It had snowed, so he was able to follow his trail. He caught up with him, but was reluctant to face him until his charioteer chided him for cowardice. They met at a ford, and Conall killed Cet in a ferocious combat that left Conall near to death himself.

He was found by Bélchú of Breifne, a Connachtman, who took him home, tended to his wounds, and planned to fight him when he was fit. But Bélchú soon regretted his honourable behaviour and asked his three sons to kill Conall as he lay in his sickbed. Conall overheard and forced Bélchú to take his place in the bed, and when his sons arrived they killed him instead. Conall then killed the three of them and took all four heads home.

After Conchobor and his son, Cormac Cond Longas, had been killed, Conall was offered the kingship of Ulster, but he refused it. In his declining years he went to stay with Ailill and Medb of Connacht, as they were best placed to look after him. Ailill was seeing another woman behind Medb's back, so Medb incited Conall to kill Ailill, something he was happy to do as Ailill had killed Fergus mac Róich. He fled, but the men of Connacht pursued and killed him.

Although his epithet, Cernach, is usually interpreted as "victorious" or "triumphant", it is an obscure word, and some texts struggle to explain it. It has been suggested that Conall may be linked with Cernunnos, the Gaulish horned god.

Unusually for a character from the Ulster Cycle, Conall appears in medieval Irish genealogies as the ancestor of the kings of the Dál nAraide and the Uí Echach Coba .