Brian Boru's Rise: Splitting Ireland With Máel Sechnaill, Then Taking It

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In 10th century Ireland the Uí Néill dynasty held the prestigious high kingship of Tara, while the Dál gCais of modern County Clare were a minor southwestern kingdom. Brian's father Cennétig and brother Mathgamain expanded it, and Mathgamain became king of Munster before rivals killed him in 976. Brian took control of Munster, forced Leinster to submit, and in 997 met the Uí Néill high king Máel Sechnaill at Clonfert, where they split Ireland north and south and exchanged hostages as diplomatic collateral. In 999 the two kings together crushed a Leinster and Dublin revolt at Glenmama.

In 1000 Brian marched north across the agreed border, and in 1002 at Athlone Máel Sechnaill submitted, making Brian high king. Brian enforced his rule with armed circuits of the north, and in 1011 made a northern king travel south to submit in person. In 1013 Leinster and Sitric Silkbeard, Norse king of Dublin, revolted; Brian besieged Dublin until winter. Sitric recruited Sigurd of Orkney and Brodir of the Isle of Man. On Good Friday, April 23, 1014, at Clontarf, the Norse broke and an incoming tide cut off their ships, but Brian's son Murchad and grandson Toirdelbach died and Brodir killed Brian in his tent.

• Hostages were royal kin kept as honored guests at a rival court, to be executed if a treaty was broken.
• According to the sagas, Sitric promised his allies the high kingship and his mother's hand in marriage.
• Sitric watched the battle from Dublin's walls beside his wife, who was Brian's own daughter.
• In 1860 the scientist Samuel Haughton calculated that high tide that day matched the medieval accounts.
• A 2018 network study of over 1,000 relationships in the Cogadh Gáedel re Gallaib found both a Norse versus Irish divide and heavy violence within the Irish side.