The Dál gCais (English: Dalcassians) were a Gaelic Irish tribe which is generally accepted by contemporary scholarship as being a branch of the Déisi Muman and became a powerful grouping in Ireland during the 10th century. They claimed descent from Cormac Cas, who is said to have lived in the 3rd century. Their ancestors are the subject of The Expulsion of the Déisi tale and one branch of their kinsmen went on to rule the petty kingdom of Dyfed in Great Britain during the 4th century; probably in alliance with Roman Emperor, Magnus Maximus.
Brian Bóruma is perhaps the best known member of the dynasty and responsible to a significant degree of carving out their fortunes. The family had carved out a powerbase on the banks of the River Shannon and Brian's brother Mathgamain mac Cennétig, became King of Munster, taking it away from the rival Eóganachta who had held it for centuries. This influence was extended under Brian who became High King of Ireland, in a series of conficts with Norse and other Irish tribes, before dying famously at the Battle of Clontarf in 1014. Following this they provided three more High Kings of Ireland; Donagh O'Brien, High King of Ireland, Turlough O'Brien, High King of Ireland and Murtagh O'Brien, High King of Ireland; but did not dominate for long.
From the 12th to the 16th century, the Dál gCais had to content themselves with the Kingdom of Thomond. There was an attempt for a while to claim the Kingdom of Desmond and thus all of Munster from the MacCarthys, but this came to nothing. For a while the Kennedys held the Kingdom of Ormond also. Some of the better known septs included O'Brien, MacNamara, O'Grady, Kennedy, MacMahon and Clancy. During the 13th century Richard Strongbow's relatives the Norman de Clares attempted to take Thomond, but the Dál gCais held firm.
It wasn't until the 16th century when the dynasty, unable to be defeated military agreed with Henry VIII Tudor to surrender and regrant, joining the nobility and the Kingdom of Ireland. They remained influential but had to endure the insult of having their land renamed County Clare. In later times, remarkable figures include writer Standish James O'Grady, who is called Father of the Celtic Revival and William Smith O'Brien who played a leading part in the Young Irelander Rebellion of 1848. Descendants of the tribe in diaspora have become notable also, including Marshal Patrice de Mac-Mahon, President of France, as well as both John F. Kennedy and Ronald Reagan, who were Presidents of the United States.
Origins and historiography; Déisi Muman vs. Deirgtine
In their own genealogies, the Dál gCais traced their line back to their eponymous ancestor and progenitor Cormac Cas, who is said to have lived in the 2nd to 3rd century. They make him a second son of Ailill Aulom from the Deirgtine, a King of Munster and Leath Moga more generally, associated in a story with the goddess Áine of the Tuatha Dé Danann during the Cycles of the Kings of Irish mythology. Cormac Cas himself was purported to be the younger brother of Eógan, founder of the Eóganachta, who would go on to rule Munster for many centuries. While this was taken on face value for a long time, later Irish scholars came to question its validity, regarding it as a politically motivated fabrication. The Dál gCais were becoming powerful in the 10th century, with Mathgamain and his brother Brian Bóruma taking the throne in Munster from the Eóganachta; claiming ancient kinship with their rivals would have boosted their legitimacy.
It is claimed by current scholarship that the Dál gCais were instead a branch of the Déisi Muman. The Déisi Muman held a vassal kingship in Munster under the Eóganachta, significant in scope, based around what is today Waterford. As time went on branches also emerged around the River Shannon, as part of the Déisi Muman moved north-westernly between the 5th and early 8th century; they were called the Déisi Deiscirt and the Déisi Tuisceart. It is from later, more northernly branch, which the Dál gCais are said to ultimately find their true ancestors. The first recorded mention of their adoption of the new name Dál gCais specifically is in the Annals of Inisfallen for the year 934, which records the death of their king Rebachán mac Mothlai.
The Déisi Muman themselves are subject of The Expulsion of the Déisi epic in the Cycles of the Kings, which is set during the time that Cormac mac Airt was High King of Ireland. The story describes the expulsion of the Dal Fiachrach Suighe; kinsmen of the Connachta and descendants of Fedlimid Rechtmar; from Tara, coming to settle in Munster after many battles. Upon becoming the Déisi Muman, one branch then sailed across to Britain in the 4th century, coming to rule Dyfed. Their presence in Britain may have been initially supported by Magnus Maximus, Roman Emperor, as part of a policy of backing Gaelic vassals to be seafaring defenders of the shores of Britain facing the Irish Sea from pirates. Eoin MacNeill has pointed out that they were not the only Irish colony in the area, with the Uí Liatháin also powerful.
High Kingship of Ireland and ending Viking influence
The adoption of the name Dál gCais and the ascent of the group to greater power, began to take place during the 10th century with internal political transition. With the death of king Rebachán mac Mothlai, the leadership of the Déisi Tuisceart shifted from the Uí Óengusso kindred to their junior relatives the Uí Thairdelbaig. It was during the time of Cennétig mac Lorcáin, who styled himself King of Thomond, that the Dál gCais began to challenge the Eóganachta; though Cennétig was defeated at the Battle of Gort Rotacháin by Cellachán Caisil, King of Munster in 944. The actual reason for this sudden surge has been much debated and one frequently discussed thesis is that it was a political scheme of the Uí Néill, intending to use the Dál gCais as proxies to further weaken the power of the Eóganachta.
Cennétig's offspring built on what their father had achieved; during his lifetime his daughter Órlaith had become Queen consort of Ireland, after she married Donnchad Donn, a High King of Ireland from the southern branch of the Uí Néill. Mathgamain mac Cennétig became the first Dál gCais to gain the kingship of Munster, after he seized the Rock of Cashel from Máel Muad mac Brain of the Eóganachta. Leading up to this he had defeated the Norse under the leadership of Ivar of Limerick at the Battle of Sulcoit in 968. After Mathgamain was captured by Donnubán mac Cathail in 976 and murdered by Máel Muad, the Eóganachta returned to the throne at Cashel for two years, but Mathgamain's younger brother Brian Bóruma, skilled in the military arts himself from the early campaigns, would desire vengeance.
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