Their known ancestors are the subject of The Expulsion of the Déisi tale and one branch of their blood-line went on to rule the petty kingdom of Dyfed in Wales during the 4th century; probably in alliance with the Roman Emperor Magnus Maximus.

The family had built a power base on the banks of the River Shannon and Brian's brother Mahon became their first King of Munster, taking the throne from the rival Eóganachta.

This influence was greatly extended under Brian who became High King of Ireland, following a series of wars against Hiberno-Norse kingdoms and the Chiefs of other Irish clans, before dying famously at the Battle of Clontarf in 1014.

During the 13th century Richard Strongbow's relatives the Norman de Clares attempted to take Thomond, but the Dál gCais held firm.

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.

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.

The Dál gCais were becoming powerful in the 10th century, with Mahon and his brother Brian Bóruma taking the throne in Munster from the Eóganachta; claiming ancient kinship with their rivals would have boosted their legitimacy.

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.

Historian C. Thomas Cairney stated that the Dal gCais along with the Déisi were tribes of the Erainn who were the second wave of Celts to settle in Ireland from about 500 to 100 BC.

[9] 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.

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.

A closely contested war of 15 years ensued, with the naval ability of the Dál gCais paying off as a truce was called by Malachy in 997, recognising Brian's overlordship of Leath Moga.

Historian C. Thomas Cairney stated that the Dal gCais were the axe-wielding foot-soldiers who formed the core of the army that defeated the Vikings in 1014.

[9] After the death of Brian, his two established surviving sons; Donagh and Teague; struggled in an internal Dál gCais rivalry for dominion.

From the Uí Thairdelbaig came O'Brien, O'Kennedy (who were Kings of Ormond), MacConsidine, MacMahon, O'Reagan, MacLysaght, O'Kelleher, Boland, Cramer, Kearney, O'Casey, Power, Twomey, Eustace, Ahearne, MacGrath, Quick, O'Meara, Scanlan, MacArthur, Cosgrave, O'Hogan, Lonergan and others.

[12] Although a junior line, the Uí Caisin continued to have a prominent role and the leader of their kindred in Thomond were the MacNamara, who as Lords of Clancullen, were the second most powerful family after the O'Brien.

[14] Other families descended from the branch of Carthann (son of Caisin) include Harley, Flood, Torrens, Stoney, Hickey, O'Hay, Clancy, Neylon and Flattery.

However, the nature of this system, known as tanistry was aristocratic ("rule by the best") in the truest sense, in that if the tribe thought a younger male member of the family was more suitable to succeed to a role than an older sibling, then they could.

[24] Famously participating in the Contention of the Bards in support of the honour of the tribe, the MacBrody sept were the principal poets and historians of the Dál gCais for centuries.

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.

[31] 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.

In two different election pamphlets, Éamon de Valera's Fianna Fáil party addressed voters as "Dalcassians", the term having become a romantic synecdoche for the Irish as a whole.