Brian O'Brien (Boru)
d. 1014
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About
Brian Boru, High King of Ireland, was born about 941 at Kincora above the Shannon, chief of the Dal gCais, and he made himself the most powerful man in the country by force, patience and a genuine gift for administration. He broke the Norse power at Clontarf on Good Friday in 1014 and was killed in his tent at the moment of victory. Every O'Brien takes their surname from him: the name means descendant of Brian. He was made King of Munster on the Rock of Cashel in 978. This family descends from him through the O'Briens of Thomond, a descent set out in the regnal pedigrees, corroborated by the Irish annals, and independently numbered by the O'Brien surname DNA study.
Research notes
✓ DOCUMENTED, DNA-CORROBORATED (O'Hart, Irish Pedigrees (1892), 'O'Brien (No.1) King of Thomond', gen. nos. as cited; corroborated by the Annals of the Four Masters and by the DC782/R-L226 O'Brien Y-DNA study (which independently numbers this same descent 105 Brian Boru -> 120 Turlogh Donn). Wired 23 Jul 2026.): **BRIAN BOROIMHE (BRIAN BORU), HIGH KING OF IRELAND** (Ard Ri), 175th Monarch of Ireland, Chief of the Dal gCais / Dalcassians, King of Thomond and of Munster; b. ~941 at Kincora, Killaloe; broke the Norse power and FELL AT THE BATTLE OF CLONTARF, Good Friday 23 April 1014 - Ireland's most celebrated High King, and the eponym of the surname O'BRIEN ("descendant of Brian"). He m. 4 times; our descent is by his 2nd wife EACHRAIDH (dau. of Ceaibhall son of Olioll Fionn), through his son TEIGE (No.106). *** THE TOP OF THE WIRED TREE ON THE GAELIC SIDE, beside Alfred the Great, Charlemagne and the Conqueror. *** DNA: the O'Brien male line carries R-L226 (Irish Type III / Dalcassian) and its subclade R-DC782, "the Brian Boru SNP" - modern testers of documented Brian-Boru descent (incl. the 18th Baron Inchiquin) confirm it. *** ABOVE HIM = NOTE-ONLY, THE LEGEND FRONTIER: his father Cinneidigh (Cennetig, No.104, King of Thomond, d.951) and the Dalcassian stem O'Hart traces through Cas (No.91, b.347) and Cormac Cas to Olioll Olum and ultimately (as was the antiquarian fashion) to Milesius and Noah - BUT modern scholarship regards the pre-Brian Dalcassian-to-Heber connection as a 10th-c DYNASTIC FICTION and the deepest reaches as myth; kept NOTE-ONLY, exactly as Cerdic/Woden above Alfred and the Carolingians past Charles Martel. Direct ancestor. Added 23 Jul 2026.