Early Irish law consisted of the accumulated decisions of the Brehons, or judges, guided entirely by an oral tradition. Some of these laws were recorded in text form by Christian clerics. The earliest theory to be recorded is contained in the Prologue to the Senchas Már. According to that text, after a difficult case involving St. Patrick, the Saint supervised the mixing of native Irish law and the law of the church. A representative of every group came and recited the laws related to that group and they were written down and collected into the Senchas Már, excepting that any law that conflicted with church law was replaced. The story also tells how the law transitioned from the keeping of the poets, whose speech was “dark” and incomprehensible, to the keeping of each group who had an interest in it.
Source: Early Irish law – Wikipedia