A fili was a member of an elite class of poets in Ireland, up until the Renaissance, when the Irish class system was dismantled by the English.
The word "file" is thought to derive from the Proto-Celtic *widluios, meaning "seer, one who sees" (attested on the Gaulish inscription from Larzac as "uidluias", which is the feminine genitive singular form), derived ultimately from the verb *widlu-, "to see". This may suggest that the filí were originally prophetic poets, who foretold the future in the form of verse or riddle, rather than simply poets.
According to the Textbook of Irish Literature, by Eleanor Hull:
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.
The fili maintained an oral tradition that predated the Christianization of Ireland. In this tradition, poetic and musical forms are important not only for aesthetics, but also for their mnemonic value. The tradition allowed plenty of room for improvisation and personal expression, especially in regard to creative hyperbole and clever kenning. However, the culture placed great importance on the fili's ability to pass stories and information down through the generations without making changes in those elements that were considered factual rather than embellishment.
In this manner, a significant corpus of pre-Christian myth and epic literature remained largely intact many centuries into the Christian era. Much of it was first recorded in writing by scholarly Christian monks. The synergy between the rich and ancient indigenous oral literary tradition and the classical tradition resulted in an explosion of monastic literature that included epics of war, love stories, nature poetry, saint tales and so forth which collectively resulted in the largest corpus of non-Latin literature seen in Europe since Ancient Greece.