Eleanor Hull’s Textbook of Irish Literature says:
The fili maintained an oral tradition that pre-dated 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.
See Early Irish literature for more details.
The ultimate accommodation of Christianity within Irish Gaelic society resulted in a strain on the resources of the Chiefs and in that they were required to provide land and titles for both fili and bishop alike. Consequently, a decision was made in the 6th century to limit the number of fili to certain families who were respected and believed to be poets as a birth right. The greatest of these families included the Ó Dálaigh (O'Daly), several of whom were accorded the rank of 'chief ollamh of poetry of all Ireland,' and O'Higgins who were hereditary filí in more than one Gaelic house such as The O'Conors, The MacDermotts, The McDonagh and O'Doherty. The hereditary poets that were a fixure of court life in medieval Ireland serving as entertainers, advisors and genealogists maintained practices of and enjoyed a similar status as the pre-Christian fili. But from the 12th century onwards, Anglo-Norman elements had increasing influence on Irish society. As Gaelic culture waned, these folk became increasingly involved with written literature and such non-native traditions as heraldry. Eventually classical literature and the Romantic literature that grew from the troubadour tradition of the langue d'oc superseded the material that that would have been familiar to the ancient fili.
See Bard for more details.
Fortunately, many manuscripts preserving the tales once transmitted by the fili have survived. This literature contributes much to the modern understanding of druids, Celtic religion and the Celtic world in general.
Besides its value to historians, this canon has contributed a great deal to modern literature beginning with retellings by William Butler Yeats and other authors involved with the Celtic Revival. Soon after, James Joyce drew from material less explicitly. Now fantasy literature and art draws heavily from these tales and characters such as Cúchulainn, Finn McCool and the Tuatha Dé Danann are relatively familiar.
Through such traditional musicians as Turlough O'Carolan (who died in 1738 and is often lauded as "the last of the bards") and countless of his less-known or anonymous colleagues, the musical tradition of the fili has made its way to contemporary ears via artists such as Planxty, The Chieftains, and The Dubliners.
Perhaps most notably,in their subject matter and techniques, the seanachie are very much the inheritors of the ancient Irish traditional of oral literature.
The modern Irish and Scottish Gaelic words for "poet" are derived from fili.
Finally, practitioners of Celtic Reconstructionist Paganism are working to reconstruct trance and visionary techniques that were used by the filid, such as imbas forosnai and aspects of the tarbhfeis ritual.