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 till the Normans. 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 (
Geoffrey Keating and the
Four Masters 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.