O'Brien is in Irish Ó Briain, from the personal name Brian. The meaning of this is problematic. It may come from bran, meaning "raven", or, more likely, from Brion, a borrowing from the Celtic ancestor of Welsh which contains the element bre-, meaning "hill" or "high place". By association, the name would then mean "lofty" or "eminent".
Whatever the initial meaning of the word, the historic origin of the surname containing it is clear. It simply denotes a descendant of Brian Ború, ("Brian of the Tributes"), High King of Ireland in 1002, and victor at the battle of Clontarf in 1014. He was a member of the relatively obscure Ui Toirdealbhaigh, part of the Dál gCais tribal grouping based in the Clare/Limerick area. Having secured control of the Dál gCais in 976, he defeated and killed the Eoghanacht king of Munster two years later, and proceeded to wage deadly war against the kingdoms of Connacht, Meath, Leinster and Breifne. Eventually he secured submission (and tributes) from all but the northern Uí Néill, the Leinstermen and the Vikings. His victory at Clontarf united all of Ireland, nominally at least, under a single leader, though Brian himself was slain. It is not surprising that Brian's harp became the model for the national emblem of Ireland.
The first individual clearly to use O'Brien as a genuinely hereditary surname was Donogh Cairbre O'Brien, son of the king of Munster, Donal Mor. His descendants spilt into a number of branches, including the O'Briens of Aherlow, the O'Briens of Waterford, the O'Briens of Arra in north Tipperary, and the O'Briens of Limerick, where the surname is perpetuated in the name of the barony of Pubblebrien. Today the name is the sixth most numerous in Ireland, widely scattered throughout the country, with particular concentrations in the above areas, as well as in the original homeland of Clare.
Unlike most other members of the native Irish ruling classes, the senior line of the O'Briens managed to retain a large part of their wealth and power, the English titles of Earls and Barons of Inchiquin, Earls and Barons of Thomond and Viscounts Clare. All the titles but the Barony of Inchiquin became extinct in 1855. The present, eighteenth Baron Inchiquin, a direct descendant of the first Baron, Murrough O'Brien, who acquired the title in 1543, is Conor O'Brien, still living in the ancestral territory of Co. Clare. The O'Brien arms symbolise clearly the royal origins of the family with the lion the regal emblem par excellence. In the crest, the arm emerging from the clouds wielding a sword is to suggest the otherworldly source of their power.
The surname has been prominent in all spheres of Irish life. The novelist and dramatist Kate O'Brien (1897-1954) suffered, like most Irish novelists of worth, at the hands of the censors in the early years of the Irish Free State. William Smith O'Brien (1803-1864) was one of the founders of the Young Ireland movement, and took a prominent part in the rising on 1848. His grandson Dermod O'Brien (1865-1945) was a leading portrait painter in Dublin for almost forty years.