The sequence begins before the Flood. Cessair, given in the text as a granddaughter of Noah refused a place on the ark, arrives with a small company; all die in the Flood except Fintan, who survives across the ages in the shape of a salmon, an eagle and a hawk and is thus available to narrate.
Partholon comes next, clears plains, introduces cattle and brewing, and his people die of plague. Nemed follows; his descendants are oppressed by the seafaring Fomorians, a recurring antagonist described in monstrous terms and never given a settlement of their own. The survivors scatter, and two branches return: the Fir Bolg, who divide Ireland into five provinces and establish kingship, and the Tuatha De Danann, who arrive from northern islands with druidic learning and four talismans, the Lia Fail, the spear of Lugh, the sword of Nuada and the cauldron of the Dagda.
The Tuatha De Danann defeat the Fir Bolg at the first battle of Mag Tuired and the Fomorians at the second. They are then defeated in turn by the Milesians, sons of Mil Espaine, who sail from Iberia, are made to withdraw beyond the ninth wave and return against druidic storm, and take the country. Ireland's name is settled by agreement with the goddess Eriu. The defeated Tuatha De Danann withdraw into the hollow hills, the sid mounds, which is how a displaced pantheon is converted into a fairy population without anyone having to call them gods.
The remainder is dynastic: king lists and genealogies running from the Milesian settlement down to historical rulers, which is where much of the text's medieval usefulness lay.