Irish folk history places the Partholonians as the second wave of inhabitants to arrive in Ireland, the first being the Fomorians. According to an early medieval text called the Lebor Gabála, a man named Parthalán (sometimes written Partholón) fled from Greece after a failed attempt to seize kingship from his brother. His mother and father were both killed during this insurrection, and Parthalán was declared an outlaw and a criminal. He took those who were loyal to him and fled Greece during a great flood, wandering the world for seven years. Some equate the great flood with the events surrounding the famous Biblical tale of Noah's Ark.

Parthalán's crimes of matricide and patricide followed him in his travels, and the bad luck they brought him kept him from being able to settle in any one place. When at last he reached Ireland, the only survivors from his original group were his wife, his three sons and their wives, and a handful of servants and retainers. One of these retainers was a brewer, and learned to brew ale from Ireland's native ferns, a practice which continues in an evolved form to this day in the brewing of heather beer.

The Partholonians found only one clear, level area on the island which was suitable for settling, and that is the plain on which the city of Dublin now stands. As previously mentioned, Ireland was nominally under control of the Fomorians at this time, who were alternately depicted as demons of the underworld and undersea. They did not live on Ireland itself, but claimed it as their territory, and confronted Parthalán there, demanding tribute from him. He refused, and the two sides fought at Magh nÍotha in the southern part of what is now Donegal. There were no casualties in this week-long battle, as the histories refer to it as a battle purely of magic and deception. Parthalán triumphed, and the Fomorians agreed that he and his descendants should inhabit Ireland as long as they wished without further interference from them.

Parthalán's bad fortune continued to plague him, however, and one common tale tells of his going hunting one day and leaving his wife in the care of his servant. His wife subsequently seduced the servant, and the two lay together and drank ale directly from the tap of Parthalán's personal keg. When Parthalán returned from hunting, he drew himself an ale and, tasting the lips of his wife and servant in it, he went into a rage and killed the servant. Dr. Daithi O hOgain, in his comprehensive encyclopedia of Irish folk history, gives the following translation of a quatrain which Parthalán's wife used to justify her actions:

"Honey with a woman, milk with a cat,
Food with a generous person, meat with a boy,
A wright where an edged tool is -
One with one, great the risk!"
Thus, the wife blames her husband for her infidelity by virtue of the fact that he left her alone with the ale and the servant, thereby providing an excuse that would live on for millennia.

After Parthalán's eventual death, his people continued to live in Ireland for over five hundred years, until a horrible plague overtook them. This plague is said to have wiped out the entire population, numbering close to ten thousand, in just one week. If there were any survivors, they perished soon after, leaving the land uninhabited for thirty years until the arrival of Neimheadh and his later descendants, the Fir Bolg.

One text, written in the late 11th or early 12th century, tells of one survivor from the race of Parthalán: Tuan mac Cairill, who through various reincarnations lives in Ireland until the sixth century when he meets the Irish abbot St. Finnen and relates to him the story of the various peoples of Ireland.