This great and illustrious sept, bearing the titles Earls of Thomond, Viscount Clare and Earls of Inchiquin, take their name from Brian Boru (941 - 1014), High King of Ireland, who was killed at the Battle of Clontarf. A very powerful and outstanding sept in Irish history, the O'Briens divided into several branches and established themselves throughout Munster. The O'Briens of Ara (north Tipperary) had as their chief Mac Ui Bhriain Ara, circa 1300; those of County Limerick gave their name to the barony of Pubblebrien; and another sept was located near Dungarvan, County Waterford. In the Annals of Innisfallen, which deal principally with the southern half of Ireland, the O'Briens appear more often that any other sept. Murrough O'Brien (died 1551) was the first Earl of Thomond, a territory comprising most of County Clare with adjacent parts of Counties Limerick and Tipperary. Charles O'Brien, sixth Viscount Clare (1699 - 1771), became a Marshal of France, and William Smith O'Brien (1803 - 1864), was one of the best known of the Young Irelanders. The first recorded spelling of the family name is shown to be that of O'Brien, which was dated circa 1055, in the "Annals of the Four Masters", during the reign of High Kings of Ireland "with opposition", 1022 - 1166. Surnames became necessary when governments introduced personal taxation. In England this was known as Poll Tax. Throughout the centuries, surnames in every country have continued to "develop" often leading to astonishing variants of the original spelling.