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O'Brien Ó Briain
Arms of O'Brien
Parent house	Dál gCais
Country	Kingdom of Munster, Kingdom of Thomond
Founder	Brian Boru
Final ruler	Murrough O'Brien, King of Thomond
Titles	Kings of Munster Kings of Cashel High Kings of Ireland Kings of Limerick Kings of Dublin King of Mann and the Isles Kings of Waterford Kings of Thomond O'Brien claim to Desmond

The O'Brien dynasty (Classical Irish: Ua Briain; Modern Irish: Ó Briain [oː ˈbʲɾʲiənʲ]; genitive Uí Bhriain [iː ˈvʲɾʲiənʲ]) was^[1] an Irish Clan and noble house of Munster, founded in the 10th century by Brian Boru of the Dál gCais (Dalcassians). After becoming King of Munster, through conquest he established himself as Ard Rí na hÉireann (High King of Ireland). Brian's descendants thus carried the name Ó Briain, continuing to rule the Kingdom of Munster until the 12th century where their territory had shrunk to the Kingdom of Thomond which they would hold for just under five centuries.

In total, four Ó Briains ruled in Munster, and two held the High Kingship of Ireland (with opposition). After the partition of Munster into Thomond and the MacCarthy Kingdom of Desmond by Tairrdelbach Ua Conchobair in the 12th century, the dynasty would go on to provide around thirty monarchs of Thomond until 1542.

During part of this period in the late 13th century they had a rivalry with the Norman de Clare house, disputing the throne of Thomond. The last Ó Briain to reign in Thomond was Murrough Ó Briain who surrendered his sovereignty to the new Kingdom of Ireland under Henry VIII of the House of Tudor, becoming instead Earl of Thomond and maintaining a role in governance.

Throughout the time that the Ó Briains ruled in medieval Ireland, the system of tanistry was used to decide succession, rather than primogeniture used by much of feudal Europe. The system in effect was a dynastic monarchy but family-elected and aristocratic, in the sense that the royal family chose the most suitable male candidate from close paternal relations—roydammna (those of kingly material) rather than the crown automatically passing to the eldest son. This sometimes led to bitter quarrels and in-family warring. Since 1542, as a part of the Peerage of Ireland, the head of the Ó Briain house adopted primogeniture to decide succession of noble titles instead.

The Ó Brian emerged as chiefs of the Dál gCais tribe from the south-west of Ireland and according to historian C. Thomas Cairney, the Dál gCais had come from the tribe of Erainn who were the second wave of Celts to settle in Ireland from about 500 to 100 BC.^[2] They were a cohesive set of septs, related by blood, all claiming descent in tradition from a common ancestor of Cas, sixth in descent from Cormac Cas.^[3] In the Annals of the Four Masters, the father of Cormac Cas was said to be Oilioll Olum, who was according to tradition King of Munster and King of Leinster in the 3rd century.^[3] Such a connection would have meant that the tribe held kinship with the Eoghanachta, a dynasty who had dominated Munster since the earliest times.^[4] While founder mythologies were very common in antiquity and the medieval world, such a connection is generally regarded as fanciful and politically motivated in the context of the rise to prominence of the Dalcassians.^[4]

Instead, academic histories generally accept the Dalcassians as being the Déisi Tuaisceart, after adopting a new name — first recorded under their newly adopted name under the year 934 in the Annals of Inisfallen.^[4] The Déisi, a people whose name means literally vassals, were originally located where today are south County Tipperary and counties Waterford and Limerick;^[5] the O'Rahilly's historical model counts them as ethnically Érainn.

The sept split into the Déisi Muman who continued to hold territory in Waterford and Tipperary, while the west Déisi controlled areas either side of the River Shannon.^[5] During the 8th century, the latter was further divided into the Déisi Deiscirt and the Déisi Tuaisceart who would become the Dalcassians.^[4][6] Prehistoric ancestors of the Déisi Tuisceart and Dál gCais may have been a once prominent Érainn people called the Mairtine.^[7]

It was during this century that the tribe annexed to Munster the area today known as Clare and made it their home. Taken from the weakened Uí Fiachrach Aidhne it had previously been part of Connacht but was renamed Thomond (Tuamhain, meaning North Munster). After gaining influence over other tribes in the area such as the Corcu Mruad and Corcu Baiscinn, the Dalcassians were able to crown Cennétig mac Lorcáin as King of Thomond, he died in 951.^[6] His son Mathgamain mac Cennétig was to expand their territory further according to the Annals of Ulster; capturing the Rock of Cashel capital of the Eoghanachta, the Dalcassians became Kings of Cashel and Munster over their previous overlords for the first time in history.^[4]

Mathgamain along with his younger brother Brian Boru began military campaigns such as the Battle of Sulcoit, against the Norse Vikings of the settlement Limerick, ruled by Ivar. The Dalcassians were successful, plundering spoils of jewels, gold and silver, saddles, finding "soft, youthful, bright girls, booming silk-clad women and active well-formed boys".^[4][8] The males fit for war were executed at Saingel, while the rest were taken as slaves.^[8]

Through much of his reign Mathgamain was competing with his Eoghanachta rival Máel Muad mac Brain.^[6] Mathgamain was only defeated in the end by a piece of treachery; he believed he was attending a friendly meeting, but was betrayed at Donnubán mac Cathail's house, handed over to his enemies and executed in 976.^[9] The crown of Munster was briefly back in the hands of the Eoghanachta for two years until Brian Boru had thoroughly avenged his brother,^[10] with the defeat and slaying of Máel Muad in the Battle of Belach Lechta.

The following year Brian came to blows with the Norsemen of Limerick at Scattery Island where a monastery was located. Whilst all parties were Christians, when their king Ivar and his sons took refuge in the monastery, Brian desecrated it and killed them in the sanctuary; the Vikings of Limerick had earlier killed Brian's mother.^[11]

Following this the Dalcassians came into conflict with those responsible for the death of Mathgamain, the Eoghanachta represented by Donovan and Molloy. A message was sent to Molloy, where Boru's son Murrough would challenge him in single combat; eventually the Battle of Belach Lechta took place where Molloy along with 1200 of his soldiers were slain. Donovan was destroyed together with Aralt, his brother-in-law and Ivar's remaining son, newly elected king of the Danes and Foreigners of Munster, in Donovan's fortress of Cathair Cuan, which Brian razed. With this Brian Boru was now the King of Munster.^[8]

Brian's rise did not go unnoticed, however; Máel Sechnaill II from the Clann Cholmáin sept of the Uí Néill, as reigning king of Mide and High King of Ireland marched an army down to Munster to send a warning to the Dalcassians. His army cut down the tree of Magh Adhair, which was sacred to the Dalcassians as it was used as their site of royal inaugurations. This sparked a conflict between Máel Sechnaill and Brian, the object of both men to be recognised as High King.

A treaty would eventually be reached between Máel Sechnaill and Brian which split the areas of influence in Ireland between them. Brian gained control over a large portion of the island's south including Dublin. The peace didn't last long as Brian used the newly acquired forces of Dublin and Leinster to spearhead an attack against Máel Sechnaill which ended in their defeat and forced Brian to reconsider pressing any further North.^{[citation needed]}

The war dragged on but Brian would eventually force Máel Sechnaill to accept his authority when northern branch of the Uí Néill clan refused to support him. Despite his fall in position Máel Sechnaill would become one of Brian's most important allies. Eventually the northern Uí Néill branch would accept Brian's rule as well, unusually for the time this was done peacefully, their submission to Brian was negotiated by the clergy rather than forced in battle.

With the most powerful Kings in Ireland now accepting Brian as the High King it was a much easier task for Brian to force the remaining Kings to submit to his rule and though it may have been tenuous he eventually was acknowledged as High King by all the rulers in Ireland.^[12]

Brian's descendants, the Ua Briain provided a further three High Kings of Ireland and exercised supremacy in Munster until Tairrdelbach Ua Conchobair, taking advantage of war between brothers Diarmait and former High-King Muircheartach, invaded Munster and split it in two in the Treaty of Glanmire (1118) granting Thomond to the sons of Diarmait Ua Briain and Desmond to the leading sept of the dispossessed Eoganacht, the Mac Cárthaigh dynasty. After the death of Domnall Mór Ua Briain, a claimant to the Kingship of Munster, they further retreated beyond the Shannon into the area of modern County Clare in the wake of the Norman Invasion. In 1276 King Edward II granted all of Thomond to Thomas de Clare, taking advantage of the feuding between Clann Taidhg and Clann Briain (whom de Clare supported). The de Clares failed in conquering Thomond, and were decisively defeated in the Battle of Dysert O'Dea in 1318, thus the Kingdom of Thomond remained outside of foreign control for a further 200 years.^[13]

In 1543 Murchadh Carrach Ó Briain agreed to surrender his Gaelic Royalty to King Henry VIII and accepted the titles Earl of Thomond and Baron Inchiquin. At his death in 1551 the Earldom passed to his nephew Donough by special remainder, and the title Baron Inchiquin passed to his male heirs through his son Dermot. The Earldom went extinct at the death of Henry O'Brien, 8th Earl of Thomond; the next heir would have been a descendant of Daniel O'Brien, 3rd Viscount Clare who had been attainded in 1691, so the title became forfeit. However, Charles O'Brien, 6th Viscount Clare, a Jacobite exile used the title Earl of Thomond, as did his son, who died childless in 1774. At the death of James O'Brien, 3rd Marquess of Thomond, the title Baron Inchiquin passed to a distant cousin and descendant of Murrough, Sir Lucius O'Brien, 5th Baronet, and was passed down to his descendants.

Key:

• = King of Dál gCais
• = King of Thomond
• = Baron Inchiquin
• =Viscount Clare
• =Earl of Thomond, Earl of Inchiquin
• =Marquess of Thomond

O'Brien Dynasty

934-942[note 1] Lorcáin 942-951 Cinnéidigh 951-953 Lachtna 953-970 King of Munster 970-976 Mathgamain 976-978 King of Munster 978-1002 High King of Ireland 1002-1014 Brian Bórumha Ua Briain 1014-1025 High King of Ireland 1025-1063 Donnchad Tadhg died 1023 King of Munster 1063-1068 Murchad King of Munster 1068-1072 King of Dublin 1072 High King of Ireland 1072-1086 Toirdhealbhach King of Munster 1086 Tadhg King of Dublin 1075-1086 King of Munster 1086-1101 1115-1118 (disputed) High King of Ireland 1101-1114 1118-1119 Muircheartach King of Munster 1114-1115 1115-1118 (disputed) Diarmaid King of the Isles 1111-1112 1115 Domhnall 1119-1138 King of Munster 1138-1142 (claimant) Conchubhar na Cathrach King of Munster 1142–1151 (claimant) 1154-1165 Toirdhealbhach 1151-1154 Tadhg 1157 Conchubhar Muircheartach King of Munster 1167-1168 (claimant) Muircheartach King of Munster 1168-1194 (claimant) Domhnall Mór Diarmaid 1194 Muircheartach Dall 1198 Conchubhar Ruadh 1198-1242 Donnchadh Cairbreach 1242-1268 Conchubhar na Siudane Tadhg Cael Uisce died v.p 1259 Clann Tadhg 1268-1276 Brian Ruadh Clann Briain 1276-1306 Toirdhealbhach Mór Donnchadh Domhnaill 1306-1311 Donnchadh 1317-1343 Muircheartach 1350–1360 Diarmaid 1311-1313 Diarmaid 1313-1317 Donnchadh 1343-1350 Brian Bán 1360–1369 Mathgamain Maonmhaighe 1375–1398 (claimant) Toirdhealbhach Maol 1369–1400 Brian Sreamhach 1400-1426 Conchubhar 1426–1438 Tadhg an Glemore 1438-1444 Mathgamain Dall 1444-1459 Toirdhealbhach Bóg 1459-1461 (claimant) Donnchadh 1459-1466 Tadhg an Chomhaid 1466-1496 Conchubhar na Srona 1496-1498 Toirdhealbhach Óg 1498-1528 Toirdhealbhach Donn 1528–1539 Conchubhar 1539–1543 Last King of Thomond 1st Earl of Thomond 1543–1551 1st Baron Inchiquin 1543–1551 Murchadh Carrach Donough 2nd Earl of Thomond [note 2] 1551-1553 Dermot 2nd Baron Inchiquin 1551–1557 Donough died 1582 Connor 3rd Earl of Thomond 1553-1581 Murrough 3rd Baron Inchiquin 1557–1573 Connor died 1603 Donough 4th Earl of Thomond 1581-1624 Daniel 1st Viscount Clare 1662-1663 Murrough 4th Baron Inchiquin 1573–1597 Donough died 1634 Henry 5th Earl of Thomond 1624-1639 Barnabas 6th Earl of Thomond 1639-1657 Connor 2nd Viscount Clare 1663-1670 Dermot 5th Baron Inchiquin 1597–1624 Connor died 1651 Henry 7th Earl of Thomond 1657-1691 Daniel 3rd Viscount Clare 1670-1691 Murrough 6th Baron Inchiquin 1624–1674 1st Earl of Inchiquin 1654–1674 Donough 1st Baronet of Leameneh 1686-1717 Henry Horatio Lord Ibrackan died v.p 1690 Daniel 4th Viscount Clare 1691-1693 Charles 5th Viscount Clare 1693-1706 William 2nd Earl of Inchiquin 7th Baron of Inchiquin 1674–1692 Lucius died v.p 1717 Henry 8th Earl of Thomond 1691-1741 Charles 6th Viscount Clare 1706-1761 9th Earl of Thomond (titular) 1741-1761 William 3rd Earl of Inchiquin 8th Baron of Inchiquin 1692–1719 Edward 2nd Baronet of Leameneh 1717-1765 Charles 7th Viscount Clare 10th Earl of Thomond (titular) 1761-1774 William 4th Earl of Inchiquin 9th Baron of Inchiquin 1719–1777 James died 1771 Lucius Henry 3rd Baronet of Leameneh 1765-1795 Murrough 5th Earl of Inchiquin 10th Baron of Inchiquin 1777–1808 1st Marquess of Thomond 1800-1808 Edward Dominic died 1801 Edward 4th Baronet of Leameneh 1795-1837 William 2nd Marquess of Thomond 6th Earl of Inchiquin 11th Baron of Inchiquin 1808-1846 James 3rd Marquess of Thomond 7th Earl of Inchiquin 12th Baron of Inchiquin 1846-1855 Lucius 5th Baronet of Leameneh 1837-1855 13th Baron Inchiquin 1855-1872 Edward Donogh 14th Baron Inchiquin 1872-1900 Lucius William 15th Baron Inchiquin 1900-1929 Murrough died 1934 Donough Edward Foster 16th Baron Inchiquin 1929-1968 Phaedrig Lucius Ambrose 17th Baron Inchiquin 1968-1982 Fionn Myles Maryons died 1977 Murrough Richard died 2000 Conor Myles John 18th Baron Inchiquin 1982-2023 Conor John Anthony 19th Baron Inchiquin 2023-present

• Irish nobility
• Irish royal families
• List of people named O'Brien
• Irish clans

• The O'Brien Clan