Fachtna Fáthach ("the wise"), son of Cas (or Ross), son of Rudraige, was, according to medieval Irish legend and historical tradition, a High King of Ireland. He came to power when he defeated the previous High King, Dui Dallta Dedad, in the battle of Árd Brestine. According to some stories he was the lover of Ness, daughter of Eochaid Sálbuide, king of Ulster, and the father of her son, Conchobar mac Nessa, the king of Ulster in the stories of the Ulster Cycle.
After he had reigned for sixteen or twenty-five years, he paid a visit to Ulster. While he was there, Eochu Feidlech, king of Connacht, raised an army and marched on Tara. With the support of the Ulstermen, Fachtna challenged him to battle. Eochu agreed, and named the battlefield as Leitir Ruad in the Corann, Connacht. During the battle Eochu surrounded and beheaded Fachtna, and became High King in his place.
The Lebor Gabála Érenn synchronises Fachtna's reign with the Roman civil war between Caesar and Pompey (49 BC) and the reign of Cleopatra (51–30 BC). The chronology of Geoffrey Keating's Foras Feasa ar Éireann dates his reign to 110–94 BC, that of the Annals of the Four Masters to 159–143 BC;
- Dictionary of the Irish Language Compact Edition, Royal Irish Academy, 1990, p. 295
- Kuno Meyer, "Anecdota from the Stowe MS. No. 992", Revue Celtique 6, 1884, pp. 173-186
- Margaret C. Dobs (ed. & trans.), "La Bataille de Leitir Ruibhe", Revue Celtique 39, 1922, pp. 1-32
- R. A. Stewart MacAlister (ed. & trans.), Lebor Gabála Érenn Part V, Irish Texts Society, 1956, pp. 296-299
- Geoffrey Keating, Foras Feasa ar Éireann 1.31
- Annals of the Four Masters M5041-5057
Dui Dallta Dedad
| High King of Ireland
LGE 1st century BC
FFE 110–94 BC
AFM 159–143 BC