From An Illustrated History of Ireland by Margaret Anne Cusack
« start... Chapter X. ...continued
One of the most noticeable peculiarities of the Brehon law is the compensation for murder, called eric. This, however, was common to other nations. Its origin is ascribed to the Germans, but the institution was probably far more ancient. We find it forbidden  in the oldest code of laws in existence; and hence the eric must have been in being at an early period of the world's civil history.
The law of succession, called tanaisteacht, or tanistry, is one of the most peculiar of the Brehon laws. The eldest son succeeded the father to the exclusion of all collateral claimants, unless he was disqualified by deformity, imbecility, or crime. In after ages, by a compact between parents or mutual agreement, the succession was sometimes made alternate in two or more families. The eldest son, being recognized as presumptive heir, was denominated tanaiste, that is, minor or second; while the other sons, or persons eligible in case of failure, were termed righdhamhua, which literally means king-material, or king-makings. The tanaiste had a separate establishment and distinct privileges. The primitive intention was, that the "best man" should reign; but practically it ended in might being taken for right, and often for less important qualifications.
The possession and inheritance of landed property was regulated by the law called gavelkind (gavail-kinne), an ancient Celtic institution, but "common to Britons, Anglo-Saxons, and others. By this law, inherited or other property was divided equally between the sons, to the exclusion of the daughters (unless, indeed, in default of heirs male, when females were permitted a life interest). The tanaiste, however, was allotted the dwelling-house and other privileges.
The tenure of land was a tribe or family right; and, indeed, the whole system of government and legislation was far more patriarchal than Teutonic—another indication of an eastern origin. All the members of a tribe or family had an equal right to their proportionate share of the land occupied by the whole. This system created a mutual independence and self-consciousness of personal right and importance, strongly at variance with the subjugation of the Germanic and Anglo-Norman vassal.
The compilation of the Brehon laws originated in a question that arose as to how the murderer of Odran, Patrick's charioteer, should be punished. The saint was allowed to select whatever Brehon he pleased to give judgment. He chose Dubhthach; and the result of his decision was the compilation of these laws, as it was at once seen that a purely pagan code would not suit Christian teaching.
 Forbidden.—"You shall not take money of him that is guilty of blood, but he shall die forthwith."—Numbers, xxxv. 31.