Forgall Monach or Manach (the dextrous, wily) is a character in the Ulster Cycle of Irish mythology. He lives at Luglochta Loga (the gardens of Lugh) in Lusk, County Dublin.
The Ulster hero Cúchulainn falls in love with his younger daughter, Emer, but Forgall is opposed to the match, ostensibly because his older daughter Fial is not yet married. He visits Ulster, disguised as the king of the Gauls, and suggests that Cúchulainn should go to Alba (Scotland) to train in arms under the warrior-woman Scáthach, hoping the ordeal will lead to his death. While he was away he offers Emer to Lugaid mac Nóis, a king of Munster, but when he hears that Emer loves Cúchulainn, Lugaid refuses her hand. When Cúchulainn returns from Scotland fully trained, Forgall still refuses to let him marry Emer. Cúchulainn storms Forgall's fortress, killing twenty-four of Forgall's men, abducts Emer and steals Forgall's treasure. Forgall himself falls from the ramparts to his death.