Before turning to the mystery genre as Peter Tremayne, Peter Berresford Ellis had written dozens of books, primarily biographical and critical works under his own name or fiction (principally war and fantasy novels) under pseudonyms. Although respected as a scholar in England and Ireland, he did not win wide international popularity until he embarked on his Sister Fidelma series in 1993 with four short stories introducing the Irish nun.
The Fidelma stories introduced the author, as Peter Tremayne, to an international audience. The Fidelma series proved especially popular in the United States, where the International Sister Fidelma Society was established in 2001. The society publishes a magazine, The Brehon, about the author and Fidelma-related matters. In September, 2006, Féile Fidelma, the first international conference on the Sister Fidelma stories, was held at Cashel, Ireland.
The Sister Fidelma stories occupy a unique position in mystery writing with their seventh century Irish setting, detailed historical context, reflections of early Irish Christianity, and a protagonist who seems both historically credible and engagingly modern in her attitudes and attributes.
An occasional reviewer has complained that Tremayne idealizes early Celtic society, but most critical responses have been positive. On the whole, the historical dimensions of the stories have been viewed as accurate and important to the success of the series.