There is reason to believe that the "Filidhs" were people who worked in society as prophetic poets and philosophers who foretold the future in verses and riddles, rather than in simple poetry. The word could also be written like this: "file".
The Filidhs kept the oral traditions of the pre-Christian Ireland alive. Telling stories by verses accompanied by music would make people better understand and remember the messages they wanted to get through.Their deeds were not only meant to entertain but also to teach.
Another group of poets was called bards, and the Norse culture had a similar function called "skalder". Snorri Sturlusson became the most famous among them for writing his "Edda".
Many of the filidh's manuscripts have survived and can tell us something about druids, Celtic religion and the Celtic world.
You can find out more about the Filidhs at the digital gaelic dictionary Dwelly-d
Under the Leaves is a series of articles about the subjects that inspired the poetry in "The Dragon Leaves".
You can buy "The Dragon Leaves" on Amazon.