The Sister Fidelma mysteries are a series of historical mystery novels and short stories by Peter Tremayne (pseudonym of Peter Berresford Ellis) about a fictional detective who is the eponymous heroine of a series. Fidelma is both a dalaigh (an advocate in the courts of the Five Kingdoms of Éireann), and Celtic nun (Ellis uses the French term "réligieuse").
Fidelma usually solves crimes in company with her partner (and eventually husband) Brother Eadulf, a Saxon monk (Ellis uses the French term "réligieux"). Though Eadulf has often been compared to Sherlock Holmes's friend Dr. Watson, he is in fact more of an equal partner to Fidelma, and usually proves essential to solving the mystery at hand (in fact, in the story of The Leper's Bell he solves one of the main problems himself). A true companion, he has saved Fidelma's life more than once.
The Sister Fidelma stories are set in the middle of the 7th century, mainly in Ireland. They are historical whodunits in the literary tradition of Umberto Eco's The Name of the Rose, the Brother Cadfael mysteries by Ellis Peters, and the Judge Dee by Robert van Gulik (inspired by Gong'an fiction and by the official of the Chinese Tang Dynasty Di Renjie). Peter Tremayne's novels are extremely complex and usually feature the interaction of several subplots involving political intrigue, personal relationships, religious conflict, or characters' desires for personal or monetary gain.