The poet and advocate for poetry, Edward Hirsch, published a best-selling book in 1999 called How to Read a Poem and Fall in Love with Poetry. That book had an addendum of poetic terms, which he later expanded to be a book of its own - A Poet's Glossary, published in 2014. Here is Hirsch's entry for Filídh.
The filídh were a professional caste of poets in early Ireland who were often credited with the supernatural power of prophecy. The words fili and filídh are etymologically connected to “seer.” These poets, who were the successors of the druids and could practice divination, were magicians and lawgivers. They were the highest-ranking members of a group called the áes dána (literally, “the people of skill, craft”). In English, the word bard usually denotes a Celtic poet, but the filídh were in fact more aristocratic and enjoyed greater privileges than the bards. Their poetry is nonetheless called bardic, since they were entrusted with an oral tradition, the full knowledge of the tribe, which predated Christianity. Their education was daunting and they spent years at a dedicated school where poetry was studied as a craft. There were seven orders of filídh; the highest grade, the ollamh, studied for twelve years. The filídh practiced an elaborate form of syllabic poetry and mastered complex metrical forms, which employed both internal and end-rhymes, consonance, alliteration, and other devices of sound. They learned by heart at least 300 poetic meters, 250 primary stories, and 100 secondary stories. They recited traditional tales and topographical lore. They also served as crucial advisors and historical chroniclers, who remembered the genealogies of their patrons. They were so bound by tradition that there is little change in their work for the four centuries from 1250 to 1650. The poet Giolla Bríghde Mac Con Midhe explained in the thirteenth century:
If poetry were to be suppressed, my people,
if we were without history, without ancient lays,
forever, but the father of each man,
everyone will pass unheralded.
Ted Hughes said that the fili “was the curator and re-animator of the inner life which held the people together and made them what they were.”