Sister Fidelma is the detective that enables Peter Tremayne to bring 7th century Ireland to life.  She is a nun, but that says way too little about her.  She is also a qualified dalaigh or legal advocate.  She informs people about that exact framing of the law, investigates problems and crimes, hears cases and passes judgment.  Having a woman in this position is not just a modern conceit of the author - the names, if not the deeds of other female justices have survived and it is known that one named Dari was the author of an important legal text.  

Being a nun in this culture was also quite different than what we are used to.  One of the constant background themes in this series is the tension between the Celtic church and the Roman church and the many differences in their practices.  Besides being interesting stories, these books educate one in many facets of Irish culture in their time period.

Hemlock at Vespers

Absolution By Murder

Shroud for the Archbishop

Suffer the Little Children

The Subtle Serpent

The Spider's Web

The Valley of the Shadow