Ailill Finn, son of Art mac Lugdach, was, according to medieval Irish legend and historical tradition, a High King of Ireland. In the Lebor Gabála Érenn, he succeeds to the throne when his father his killed by Fíachu Tolgrach and his son Dui Ladrach. He rules for nine years. Two years into his reign, Fíachu Tolgrach is killed in battle against Airgetmar, son of Sírlám. The men of Munster, led by Ailill's son Eochu and Lugaid, son of Eochu Fíadmuine, then drive Airgetmar into exile overseas. After seven years Airgetmar returns to Ireland and kills Ailill with the help of Dui Ladrach and his son Fíachu, but is unable to seize the throne, which is taken by Eochu.
However, in Geoffrey Keating's Foras Feasa ar Éirinn and the Annals of the Four Masters Fíachu Tolgrach succeeds to the throne after killing Art, and is later killed by Ailill, who then takes the throne, and rules for nine or eleven years, before being killed by Airgetmar and succeeded by Eochu.
The Lebor Gabála synchronises his reign with that of Artaxerxes II of Persia (404-358). The chronology of Keating's Foras Feasa ar Éirinn dates his reign to 586-577 BC, that of the Annals of the Four Masters to 795-786 BC.