Gíallchad, the son of Ailill Olcháin, son of Sírna Sáeglach, was, according to medieval Irish legend and historical tradition, the 37th High King of Ireland.
Gíallchad took power after killing his predecessor, and the son of his grandfather's killer, Elim Olfínechta, in the battle of Comair Trí nUisce.
Gíallchad reigned for nine years, before being killed by Elim's son Art Imlech at Mag Muaide.
The Lebor Gabála Érenn synchronises his reign with that of Phraortes of the Medes (665–633 BCE).
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