Cormac Cas was King of Thomond around the fifth century and he spawned a tribal grouping known as the Dál gCais or Dalcassians which dominated Munster until the final suppression of the old Gaelic order in the seventeenth century. Twenty-three generations later and in direct descent from Cas we find Cumara, Chief of Maghadhair in county Clare. Cumara is a contracted form of Conmara - hound of the sea. His son, Domhnall, who died in 1099, adopted the surname Mac Conmara, or son of Cumara, thus becoming the very first MacNamara. The name has survived relatively unmodified as MacConmara in Irish and Mac (or Mc) Namara in English.

The sept of MacNamara was, after the O'Briens, the most important and powerful of the Dalcassians of Thomond. They were hereditary marshals to the O'Briens and had the privilege of inaugurating their chief who was, of course, often a king. There was frequent intermarriage between these two strong families. The sept was originally confined to a small territory, but by the end of the eleventh century they had become lords of Clancullen (which comprises a great part of East Clare) and they are so described by the Four Masters many times at various dates between 1099 and 1600. The sept in due course became two - the chief of West Clancullen (barony of Bunratty) being MacNamara Fyne (i.e. fionn, fair), and the chief of East Clancullen (baronies of Upper and Lower Tulla) MacNamara Reagh (i.e. riabhach, swarthy or grizzled). They earned a reputation as builders and are recorded as having built forty-two castles, fifteen fortresses and several friaries. Macmiccon MacNamara Fionn received a papal bull authorising him to install Friars Minor in Quin Abbey, near Ennis, which he built in 1402. Many of the family lie buried in the shadow of the now roofless abbey.
Like their ancient fortresses, the MacNamara seats are all in County Clare. From the woods around Cratloe Castle, built in 1610, came the oaks for London's old Westminster Hall and the royal palace in Amsterdam. They were to a great extent dispossessed in the Cromwellian debacle, but one family, resident until quite lately at Ennistymon, became Protestants and were extensive landlords up till the Land Act of 1903.
The history of Clare is full of the name MacNamara. Half a century before America was discovered, John Macmiccon H MacNamara, Lord of Clancullen and High Chieftain of the Dalcassians, completed the building of Bunratty Castle begun by his father, Sioda, and planned his dream castle at Knappogue.