Fiacha (earlier Fíachu) is a name borne by numerous figures from Irish history and mythology, including:
Fiacha Cennfinnán, High King of Ireland in the 16th or 20th century BCFiacha mac Delbaíth, High King in the 14th or 18th century BCFiacha Labhrainne, High King in the 11th or 15th century BCFiacha Finscothach, High King in the 10th or 14th century BCFiacha Finnailches, High King in the 9th or 13th century BCFiacha Tolgrach, High King in the 6th or 9th century BCFiacha Sraibhtine, High King in the 3rd century BCFiacha Finnfolaidh, High King in the 1st century ADFiacha Muilleathan, a king of Munster in the 3rd century ADFiachu mac Néill, son of Niall of the Nine Hostages