Ailill (or Oilioll), son of Slánoll, was, according to medieval Irish legend and historical tradition, a High King of Ireland; scholars now believe these kings to be a pseudohistorical construct of the eighth century AD, a projection into the distant past of a political entity which did not become a reality until Maelseachlainn I. He took power after killing his cousin Berngal. He ruled for twelve, fifteen or sixteen years, according to various versions of the Lebor Gabála Érenn^[1] (Geoffrey Keating^[2] and the Four Masters^[3] agree on sixteen) before he was killed by Sírna Sáeglach, a great grandson of Rothechtaid mac Main. The Lebor Gabála synchronises his reign with that of Deioces of the Medes (694-665 BC). The chronology of Keating's Foras Feasa ar Éirinn dates his reign to 831–815 BC, that of the Annals of the Four Masters to 1197–1181 BC.