Courtesy of your friend and mine, Wikipedia:

“The file is to be regarded as in the earliest times as combining in his person the functions of magician, lawgiver, judge, counsellor to the chief, and poet. Later, but still at a very early time, the offices seem to have been divided, the brehons devoting themselves to the study of law, and the giving of legal decisions, the druids arrogating to themselves the supernatural functions, with the addition, possibly of some priestly offices, and the filí themselves being henceforth principally as poets and philosophers. The division seems to have already existed in Ireland at the time of St Patrick, whose preaching brought him into constant opposition with the druids, who were evidently, at that time, regarded as the religious leaders of the nation, though there does not seem to be much sign that they were, as they undoubtedly were, even at an earlier age in Britain and Gaul, sacrificing priests.”