Emerging from the affair battered but unbowed, she applied herself wholeheartedly to her studies and eventually qualified as an anruth, the second highest rank after an Ollamh in the Irish legal system.
After her studies were complete, on the advice of her mentor Abbot Laisran of Durrow Fidelma became a nun (referred to throughout the series as a "religieuse"), joining St. Brigid's mixed abbey at Cill Dara (Kildare).
In 666 when Fidelma was called upon to investigate the finding of a headless body in an abbey well, she also became involved with the mystery of a deserted Gaulish ship on which (she learned) Eadulf had been traveling to Cashel as an emissary of Theodore of Tarsus, the new Archbishop of Canterbury, and from which he and the entire crew had been taken captive (see The Subtle Serpent).
Eadulf then convinced Fidelma to accompany him to Canterbury and then, after their business with Archbishop Theodore had been accomplished, to visit his former home of Seaxmund's Ham (see Smoke in the Wind, "The Lost Eagle" in the short story collection Whispers of the Dead and The Haunted Abbot).
After uncovering both the person responsible and a plot to assassinate the Breton king (see The Dove of Death), Fidelma decided to renounce her religious vows and become full-time legal advisor to her brother, a decision she believed Eadulf would support.
The murder of Brother Donnchad at the abbey of Lios Mor led Colgu and Abbot Segdae to send Fidelma to investigate but on the condition that Eadulf accompany and assist her (see The Chalice of Blood).
The would-be assassin was killed, but the initial investigation into the motive for the attack (originally handled by Deputy Chief Brehon Aillin) seemed to point towards the Uí Fidgente, and so, with the approval of Colgu's tanaiste (heir apparent) Finguine, Fidelma and Eadulf traveled into enemy territory to continue their investigation and uncovered not only the reason for the attack on Colgu but also a plot against the Uí Fidgente from within their own ruling family (see Atonement of Blood).
In November 671, just before the eve of the feast of Samhain, Eadulf and Aidan discovered a man murdered in an unlit pyre, dressed in the robes of a religieux and killed by the ritualistic "three deaths".
When a strange woman known as "Brancheó" appeared in a raven-feather cloak foretelling of the ancient gods returning to exact revenge upon the mortal world, she was quickly branded a suspect.
In their search for the killer, Sister Fidelma and Eadulf discovered that their investigation was linked to a book stolen from the Papal Secret Archives which could destroy the New Faith in the Five Kingdoms (see Night of the Lightbringer).
As 671 drew to a close, Fidelma traveled to the Abbey of Finnbarr on a personal secret mission to question the abbot, but found that he had been murdered and the suspect, a young girl, had fled.
In February, Fidelma and Eadulf were traveling from the Abbey of Lios Mhor back to Cashel when they reached the village of Cloichin just in time to prevent the lynching, presided over by Brother Gadra (a firm adherent of the Penitentials), of a man accused of murdering a local farmer, his wife and two sons.
Determined that the man, a foreign vagrant, should be given a fair trial, Fidelma and Eadulf remained in the area, but events, including a claimant to the murdered farmer's inheritance, began to spiral into a far more sinister pattern that would cost more lives (see Blood in Eden).
In Spring 672, the body of Brother Brocc, who had been traveling with Princess Gelgeis, King Colgu's betrothed, on a secret mission, was found and brought to the Abbey of Gleann Da Loch.
While trying to solve this mystery and as rumors of an attempt to overthrow Colgú began to spread, Fidelma and Eadulf also had to contend with news that a plague ship had landed at a nearby port (see The House of Death).
Because of the death of her parents at an early age, Fidelma grew up quite independent and self-reliant; at times she refuses to delegate gathering of evidence to anyone, even Eadfulf (although she does admit that Eadulf's "mind was just as sharp and penetrating as her own."
(The Chalice of Blood) However, in spite of her self-confessed shortcomings, she has proven herself to be devoted to family and friends and a caring and loving wife and mother (although she does wish that her duties as a dalaigh didn't cause her to be away from her son so often and for so long).
Eadulf was converted to Christianity by an Irish monk named Fursa sometime before the novels begin and subsequently educated in Ireland, studying first at Daru (Durrow) then medicine at the great medical school of Tuaim Brecain.
A serious emotional and physical breach was made between them when Fidelma announced her decision to renounce her religious vows and Eadulf sadly realized that he could not change her mind (see The Dove of Death and The Chalice of Blood).
Tremayne uses Brother Eadulf's status as an outsider to the Celtic communities in which many of his and Fidelma's cases take place to provide explanations about legal and cultural matters to his readers.
Being a foreigner, Eadulf's status in Ireland is originally that of cu glas (which translates as "grey dog"), meaning an "exile from over the sea" and a person without legal standing or honor price (for a definition of this term, see "Status" in Early Irish Law); however, his rank as techtaire (emissary or ambassador) between Archbishop Theodore and Fidelma's brother King Colgu gave him a high honor price of eight cumals (a cumal being the value of three cows) under Irish law (see Our Lady of Darkness) and since his marriage to Fidelma (recognized and approved by her family) he is now considered a deorad De (an "exile of God") and has an honor price of half that of Fidelma's but he is not entitled to make legal contracts without her permission (she is also responsible for any debts that he might incur) or have any legal responsibility in the raising of Alchu.
Brother Eadulf is a stolid man who provides a much-needed stability to Fidelma during emotionally difficult cases (he knows that her insecurity stems from both her parents dying when she was very young).
His medical knowledge and assistance is often very valuable as well (especially in The Devil's Seal when he must perform an emergency amputation), and Fidelma has often admitted that Eadulf has an uncanny ability to see the obvious that she has overlooked.
In The Seventh Trumpet, he demonstrated his own powers of deduction to such a degree that Fidelma remarks, "Every day, you become more and more a Brehon"; in The Chalice of Blood, she tells him: "As far as I am concerned, without you, your advice, your ability to analyse, I would not have succeeded in many of the investigations we have undertaken ... you will forever be my soul-mate, my anam chara, and if you go my soul will die."
In the course of the series, Sister Fidelma journeys to many different parts of Western Europe, including Ireland, Wales, Northumbria, Hispania, Brittany, Francia and Rome.