The First Battle of Magh Tuiredh occurred when the Tuatha Dé Danann invaded Ireland, then ruled by the Fir Bolg, and attempted to conquer it. The Fir Bolg themselves conquered Ireland 37 years before, inhabited mostly by Fomorians.
Nemed and his children arrived in Ireland several generations before but left for Greece to escape oppression from the primeval Fomorians. The Fir Bolg, descendants of Nemed, returned to the land and conquered it, dividing it into five provinces;Connacht, Leinster, Ulster and North and South Munster. Nine Fir Bolg High Kings ruled over Ireland in 37 years, until the coming of the Tuatha Dé Danann.
The Tuatha Dé Danann, descendants of Nemed from another branch, arrived in Ireland, led by Nuada, and burnt their ships. They approached the Fir Bolg, who knew of their coming due to a prophetic dream of their king - Eochaid mac Eirc. The Tuatha Dé Danann give the Fir Bolg two options; give battle or cede half of Ireland. The Fir Bolg, championed by Sreng, choose to fight.
The Tuatha Dé Danann fight the Fir Bolg for four days, with heavy casualties; Eochaid mac Eirc is killed by the Morrigan and Sreng cuts off Nuada's right hand. The Tuatha Dé Danann gain ground and give the Fir Bolg a truce. They allow the Fir Bolg three choices; leave Ireland, share the land with the Tuatha Dé Danann, or continue fighting. The Fir Bolg choose to continue the battle. Sreng attempts to challenge Nuada to single combat, refusing to equalise the conditions by using one hand. The Tuatha Dé Danann however, decide to allow the Fir Bolg to keep the province of Connacht.
The Tuatha Dé Danann win but Brigid decreed that no blemished individual may rule as high king. This bars the one-armed Nuada from kingship, so the Tuatha Dé Danann elect Bres, half-Fomorian (through his father Elatha) and half-Tuatha Dé Danann (through his mother Eriu). Bres forces the Tuatha Dé Danann to work for the Fomorians until Nuada's hand is replaced by Dian Cecht, the god of physicians, and Bres is exiled. Bres later leads a Fomorian force against the Tuatha Dé Danann in the Second Battle of Magh Tuiredh.