Little is known about Deirdre's early life. According to oral tradition, she was born to Christian parents and quickly developed an ability as a child for "second sight". However, alarmed at her gift and believing it to be the work of the devil, they abandoned her in the forest when she was about seven or eight.
Wandering through the woods, Deirdre found an aging and infirm druid, who raised the child and taught her the pagan arts of medicine in order to help care for him; through these means, his new apprentice was able to prolong his already-extensive life by many years. After her teacher's passing, Deirdre continued her self-taught studies, and in time, soon became well known among Ireland's pagan communities as both an exceptionally powerful seer and a healer.
As an adult, Deidre moved to a hut in Asfinn within the Kingdom of Connacht and upkept her druidic practices. Eventually, she was introduced to Ciara ingen Medba, a fellow druid who had joined the cult known as the Children of Danu which sought to preserve the druidic ways from the ever-expanding influence of Christianity, and the two women became fast friends. When the cult turned away from its initial purpose and began adopting more fanatical and violent methods, Ciara left and sought out Deidre to continue her studies. After Ciara's appointment as High King Flann's Ollamh Érenn, Deidre was disappointed her friend was leaving the forest and its ways to serve a Christian king, but maintained their friendship and believed Ciara would one day return.
Assisting Ciara and Eivor
In early 880, Deirdre met the Viking jarlskona Eivor Varinsdottir of the Raven Clan, who was visiting Ireland to see her cousin Bárid mac Ímair but had been unknowingly embroiled in a greater plot in the country when Bárid was framed for poisoning Flann's army. Knowing Deirdre would be able to make a cure, Ciara took Eivor to the priestess and explained the matter. Quickly assessing the situation, she said that the cure required the mystical powers locked away in bloody amber which could be collected from any member of the Children of Danu and was rumored to come from the mythical tree Eó Mugna. With this information and working off a lead that had gone cold from a late friend of the druids, Eivor sought out and killed the cult member known as "The Wren" and retrieved their amber shard.
As she made the potion, Deirdre informed Eivor that collecting all the amber from the Children of Danu would "give [her] the force of the blazing sun". Intrigued, Eivor hunted down and systematically eliminated the rest of the cult and gathered their amber shards. By 882, Eivor returned again to Deidre's hut, delivering the minerals and the druid used them to make a psychoactive mixture which sent Eivor on a hallucinogenic vision where she fought and defeated a manifestation of the Fomorian giant named Balor for possession of the legendary spear Gae Bolg, which had been wielded by the hero Cú Chulainn.
Personality and characteristics
Deirdre was an incredibly learned individual in the pagan arts; so thorough was her knowledge in healing that she was able to immediately identify the culprit behind the affliction plaguing Flann's army, and the elements necessary for the cure, all within moments of being told the symptoms. A powerful seer as well, she had a strong connection to the Celtic Otherworld, commenting to Ciara and Eivor upon their visit that she had hoped the omens she had read were mistaken as to the pair's purpose in Asfinn. In her strongest display of her second sight, Deirdre proved astute enough to correctly determine Eivor's identity as a reincarnaiton of the Norse god Odin, twice stating that Eivor should remember having fought Balor, despite having neither met the woman before nor shown any prior knowledge of the Isu.
Behind the scenes
Deirdre Na Linni, a character in the Assassin's Creed: Valhalla DLC expansion Wrath of the Druids, shares her first name with the legendary figure Deirdre, whose tragic story of her beauty causing untold bloodshed is recorded in the Ulster Cycle.