Brian Boru(c. 941–1014)

Brian Bóramha mac Cennétig was born at Cinnéide's seat at Killaloe on the western shore of Lough Derg around 941, twelfth son of Cennétig mac Lorcáin, chief of the small Dál gCais kindred of north Munster, and Bé Binn of the southern Uí Lugdach. The Dál gCais of the period were a third-rank Munster tribe holding the small territory of Tuadhmumhain (Thomond, modern east Clare) under the overlordship of the Eóganacht Cashel dynasty that had held the kingship of Munster for the previous five hundred years. Brian was raised at Killaloe, served his apprenticeship under his elder brother Mathgamain in the long Munster guerrilla war against the Norse of Limerick through the 960s, succeeded Mathgamain on his assassination by the southern Eóganacht in 976, and within five years had taken the kingship of all Munster from the Eóganacht and established Cashel as his royal seat.

Through the 980s and 990s he extended the Dál gCais overlordship outward from Munster across the southern half of the country, took the kingship of Leinster in 996 from the Uí Néill claimant Máel Mórda mac Murchada, and in successive campaigns against Máel Sechnaill mac Domnaill, High King of the Uí Néill at Tara, forced the partition of the country into the two-king system of 998 by which Brian held the south (Leth Moga, the half of Mogh Nuadat) and Máel Sechnaill the north (Leth Cuinn, the half of Conn). The two-king partition broke down in 1002 when Brian marched a Munster-Leinster army to Tara and forced Máel Sechnaill's submission and the surrender of the high-kingship; Brian was inaugurated High King of Ireland at Tara in 1002 in his sixty-first year, the first non-Uí Néill to hold the office in five centuries.

He ruled Ireland as High King from 1002 to 1014 with a competence and a unifying ambition that no previous Gaelic king had shown. He commissioned the surviving archives of the Cashel monastery (the early text of the Sanas Cormaic and parts of the Lebor na hUidre derive from the Cashel scriptorium of his period), refounded a string of monasteries across Munster that had been raided by the Norse, paid the recompense for the books that had been lost, and conducted in 1005 the formal circuit of the country (the rí for-rí ríg Banba, the king-of-kings of Ireland) in person from Cashel through Connacht, Ulster and back to Tara, taking hostages from every provincial king as the formal acknowledgement of the high-kingship.

In 1013 the Norse king of Dublin Sitric Silkbeard and the Leinster king Máel Mórda combined in revolt against the high-kingship and summoned in support a Norse expeditionary force from Orkney, the Isle of Man and the Hebrides under Sigurd of Orkney. Brian marched a great Munster-Connacht-Ulster army to Dublin in the spring of 1014, and on Good Friday the twenty-third of April 1014 met the combined Norse-Leinster army on the level ground at Cluain Tarbh (Clontarf) outside Dublin city. The battle ran the whole of Good Friday, ended in the total defeat of the Norse-Leinster army (Sitric's Norse fled to the longships and Sigurd of Orkney fell on the field), broke the political power of the Norse over Dublin for the next century and a half, and ended in the death of Brian himself in his tent late on the Friday afternoon at the hand of the fleeing Norse Bróthir of Man.

He was buried at Armagh Cathedral on the north-east corner of the chancel, the inscription on his original grave-slab still survives. He left no settled high-king successor (his son Murchad had died at Clontarf with him), and the high-kingship reverted within a decade to the Uí Néill of Tara. The kingdom Brian had unified did not hold, but the unifying ambition he had set the precedent for was carried by the high-kings with opposition who followed him through the next century and a half, and the title Ard-Rí Éireann (High King of Ireland) remained the constitutional aspiration of every Irish ruler down to the Norman invasion of 1170. The O'Brien surname is in direct male-line descent from Brian (the prefix Ó Briain meaning grandson of Brian, applied first to his grandson Donnchad mac Briain in the late eleventh century); the O'Brien line held the kingship of Thomond at Killaloe and Clare-Limerick continuously through the next six hundred years to the Earldom of Thomond. The Brian Boru harp at Trinity College Dublin (eleventh-century, attributed by tradition to Brian) is the harp on the coat of arms of the Republic of Ireland and on every Irish coin since 1928. The O'Brien name in modern Irish historical memory carries the weight of the high-kingship and the Clontarf victory.