The Irish population was divided into a large number of groups, each of which was a "clan." At the head of each clan was a chief. The clan and the chief considered themselves to be of one blood, a great family. Each clan occupied a definite stretch of country and was in fact the population of its territory. The clan was a miniature nation. That, I think, is a fair summary of the prevailing notions as to the basis of what is called the clan system. Some writers prefer to say "tribal system." I have been reproached with avoiding the word "tribe." I have avoided it, and for two reasons; first, because some have used it in so loose a sense as to make it meaningless; and second, because others have used it with the deliberate intent to create the impression that the structure of society in Ireland down to the twelfth century, and in parts of Ireland down to the seventeenth century, finds its modern parallel among the Australian or Central African aborigines. Already, in reference to the law of succession, I have mentioned the _deirbfine_, the Irish legal family of four generations, a man, his sons, grandsons, and great grandsons. O'Donovan calls this family a tribe. I told how, in the battle of Caiméirghe in 1241, Brian O'Néill secured the kingship of Tyrone for himself and his line by cutting off his rival MagLochlainn and ten men of MagLochlainn's _deirbfine_. Here the word _deirbfine_ has a very special and technical importance; but the student who has to rely on the official editorial translation misses the whole significance of the Irish term. The translator of the Annals of Ulster renders the passage thus: "The battle of Caiméirghe was given by Brian O'Neill and Mael-Sechlainn O'Domnaill, king of Cenel Conaill, to Domnall MagLochlainn, to the king of Tir-Eogain, so that Domnall MagLochlainn was killed therein and ten of his own tribe around him; and all the chiefs of Cenel-Eogain and many other good persons likewise. And the kingship was taken by Brian O'Neill after him."