From Irish Roots, 1996, #3, p 22, col 2:
Sometime between 1206 and 1216 Donnchadha Cairbreach O'Brien established his capital in Ennis - now the principal town in Clare. In 1247 this same O'Brien gave shelter to some wandering friars and they proceeded over the years to build the magnificent Ennis Abbey (now a ruin). The Inchiquin tomb here houses the bodies of King Turlough O'Brien who died in 1306, Murrough who died in 1551, and the later Barons of Inchiquin. In 1460 Bishop Donnchadha O'Brien of Killaloe ( now the cathedral town of Clare) was killed here by Brian O'Brien.

He was King of Thomond from 1210-1242.

From chart of Hon. Donough O'Brien "Descendants of Dermod O'Brien & Donough O'Brien": See Dictionary of National Biography, v41, p312; built Ennis Abbey in 1208, also Clare Castle, Co Clare; an equestrian picture of whom is now in the Hall at Dromoland Castle; built 80 Churches, Abbeys and Chapels. One of the first he build was the Abbey of Ennis in 1208 A.D. for Franciscans. He built and fortified Clare Castle. He erected an Academy or Seminary for learning at Clonroad near Ennis where 600 scholars and a great number of monks were supported at the expense of the O'Brien family, till Henry VIII dissolved the Abbeys and Seminaries.

From book "Kings and Queens of Britain" by David Williamson, p234:
King of Thomond 1239-1242

King John landed in Ireland with a large army in 1210. To secure his support Donogh O'Brien, the prince of Thomond, entered into an alliance with King John, agreeing to pay him 1,000 marks and a yearly tribute of 100 pounds. In return Donogh was acknowledged by King John as King of Thomond against every claimant.13

The History and Topography of the County of Clare by James Frost Part I. Topography of Thomond Chapter 3. Burren, or Corcomroe East
. . .about the year 1200 O’Brien built a circular earthen fort at Clonroad.

The History and Topography of the County of Clare by James Frost Part I. Topography of Thomond Chapter 4. Ui Caisin
Moyry Park is situate in the townland of Toonagh, parish of Clooney. The tree stood there beneath which the O’Briens were inaugurated as kings of Thomond.

He surrendered to King John and was regranted his kingdom under the English crown 1222. He founded the abbeys of Corcomroe, in the barony of Burren, county Clare; Killcooley, in the parish of Slievearadh, county Tipperary; Galbally, in the parish of Galbally, barony of Costlea, county of Limerick; and the Franciscan Monastery at Ennis, county Clare.9

When Dónal Mór died in 1194 his kingdom was weakened by rivalry between his three sons, and the Normans exploited this weakness to their own advantage. Murtagh Dall, the eldest son, was blinded by the Normans, then Conor Roe, the second son, was dethroned and murdered in 1198 by his nephew. Donagh Cairbreach, the third son, invited his brother-in-law de Burgh and other Normans to help him suppress the McNamara and O'Quin clans who were in revolt against his kingship. In return for this support, Limerick city was handed over to the Normans and grants of land were given in Limerick and Tipperary. During this time the Butlers and other Normans were pushing in to eastern Thomond (north Tipperary, east Limerick and south Offaly) so that by the year 1200 the kingdom of Thomond was greatly reduced and was almost co-terminus with the modern county Clare. Fortunately, the river Shannon was a natural barrier protecting the remaining O'Brien territory from further Norman expansion, but only for a while. Reflecting the changed political situation, Donough Cairbreach moved his capital to Clonroad (Ennis) around this time.

In the meantime, Prince John, Lord of Ireland, had visited Ireland in 1185 and he granted 5? cantreds (baronies) of Thomond to Theobold Walter (Butler). This land was in east Clare, but the Butlers were not able to enforce this claim. By 1199 further grants had been made by King John. Arnold Keating received Tradaree (Bunratty lower), while the King retained for his own use the three cantreds of Corcobaskin (i.e. Moyarta, Clonderlaw and Ibrickan). Keating later exchanged his claim of Tradaree for Clonderlaw. Another Norman baron Thomas Fitzmaurice was rewarded with a grant of territory, worth 5 knights' fees, on the northern bank of the Shannon, near Limerick. However, these claims were not recognised by King Donogh Cairbreach until the year 1210 when King John visited Ireland and Donough submitted to him at Waterford. Donough acknowledged his status as tenant-in-chief, but Tradaree, Moyarta, Clonderlaw and Ibrickan were placed at the disposal of the Norman justiciar (chief governor) of Ireland, John de Gray, Bishop of Norwich. Although de Gray tried to colonise these areas he failed, due no doubt to O'Brien opposition, and Donough rented these lands from de Gray at a nominal sum, allowing some Norman settlers into the county. Donogh Cairbreach ruled as King of Thomond until his death in 1242. He got a re-grant of the crown lands of Tradaree and Corcabaskin in 1222 for an annual rent of £100. This lease was renewed in 1228 and, even though his brother Murtagh bid against him, Donogh retained possession of these lands at a higher rent. In general, Donogh Cairbreach was loyal to the English interest and he assisted his Norman brother-in-law Richard de Burgh in his campaign against the O'Connor's, King of Connaught, in the 1230's.