Peter Beresford Ellis is much more than just a mystery writer. He's a historian, a bard, a practicing druid and one of the great champions of Celtic language and culture. While maintaining an active academic career and producing numerous historical texts on the history of the Irish and other Celtic groups, he began a modest second career as a writer of horror and fantasy in the 1970s, writing under the name Peter Tremayne. He stuck with that pseudonym in the 1980s when he branched out into writing historical mysteries set in ancient Ireland, and there he found his first major commercial success as a writer.

The series of novels centers on the character of Sister Fidelma, a young religieuse from the monastery of St. Brigid of Kildare who is also a Dalaigh of the Irish Brehon court, a position which combines elements of lawyer, investigator and advocate and which carries with it considerable respect and social status within that society. The stories take place during the 7th century, a time when conflict was brewing between the Irish version of Christianity and the version promoted by the increasingly powerful Roman church.

The Irish had been the first to convert much of Western Europe and had an ancient tradition going back to the time of the apostles, entirely separate from the Petrine tradition in Rome, with different rituals and practices much more akin to those of the early church and heavily influenced by the Hellenistic culture of the early Christian era. The rivalry between the two churches is a recurring theme in the novels, as is the unique character of Irish culture in that era, which was much more literate, politically sophisticated and egalitarian in many ways than most of 'dark age' Europe.

Against this background of religious strife and the cultural struggle between Irish intellectualism and the dynamic aggressiveness of the dominant European tribes like the Saxons and Franks, Sister Fidelma finds herself in tense situations where she has to use her wits, her deductive abilities and her knowledge of both law and human nature to solve mysteries on whose outcome the fate of peoples and nations often rests. In this she is aided by the somewhat headstrong, but very able Saxon monk Brother Eadulf with whom she develops a close personal relationship.

The historical element in these novels is very strong, in the tradition of Umberto Eco's Name of the Rose, and Tremayne goes into great detail on the unique institutions of this transitional period, the specifics of the cultures and societies, and the personalities of historical characters who play roles in the stories. You learn a lot of things you might never have suspected about life in the 'dark ages,' such as the surprisingly high legal status of women in Ireland, the fact that in that era both the Roman and Irish churches allowed priests and monastics to marry, and the common existence of religious communities where men and women lived together, often cohabiting and even raising children while in holy orders. As mysteries the books are challenging, as history they are informative, and as little bits of human melodrama they are engaging.