Prior to the adoption of common law in Ireland, a native legal system, known as Brehon law, had applied throughout the country. This legal system dated from Celtic times and was passed down orally from generation to generation. It was written down for the first time in the seventh century and survived until the seventeenth century when it was finally replaced by the common law. The Brehon law system was highly complex and sophisticated. Rights were accrued based on societal status and punishment / restitution was based on the status of the person against whom an offence was committed. The legal system was administered by judges but the legal system was essentially self-enforcing with no prisons or police force. This paper will describe the roots of the Brehon legal system and its primary actors and will compare it to the common law system. It will analyse its main facets and subjects and will trace its development through Irish history up until it was finally supplanted as the legal system of Ireland by the common law in the seventeenth century.