The next group of people to arrive in Ireland, were the Nemedians. The Nemedians arrived 30 years after the extinction of the Partholanians. They probably come from the west in the Land of the Dead, or else from Spain. This group sailed for a year and half, wandering the sea with a fleet of 32 ships, carrying less than a thousand persons. Only one ship would survive the journey, including Nemed as their leader, and Nemed's four sons.

Nemed was a descendant of Japheth, the son of the biblical Noah. Nemed was married to Macha; a woman associated with Morrígan (Morrigan), from the race called Tuatha Dé Danann.

The Nemedians were the descendants of Nemed and his small group of survivors managed to gradually repopulate the isle. However, the Nemedians also had deadly encounters with the Fomorians, such as the Partholanians did earlier. Though the Nemedians were at first successful against the Fomorians, with four decisive victories, a pestilence decimated the population, until less than two thousand Nemedians survived.

The Nemedians had to suffer from Fomorian tyranny and oppression, paying heavy tributes to their overlords. Later three Nemedian chieftains led their people in revolt. They attack the Fomorian stronghold of Tory Island. Though the Nemedians managed to kill one of the Fomorian kings and captured one of the towers, the Nemedians was almost totally annihilated, when the Fomorians received reinforcement. Only thirty Nemedians survived the battle.

These survivors fled from Ireland and the Nemedians as we know them, were never heard from again. Fergus Lethderg fled with his son Britain Máel to Alba (Scotland), where the whole island was named after Britain, Nemed's grandson. While Semeon son of Erglan son of Beoan son of Starn son of Nemed had fled to Greece, where were subjugated and became slaves. The descendants were known as the Fir Bolg, who would later return to Ireland.

Another group of Nemedians migrated to the islands of Northern Greece. Iobath son of Beothach son of Iarbanel son of Nemed had brought his followers to these islands, where they became known Tuatha De. Later when they migrate back to Ireland, they became known as Tuatha De Danann.