Peter Tremayne's Sister Fidelma series (written by Celtic scholar Peter Berresford Ellis, Tremayne is a pseudonym). They are a series of detective novels about a 7th century AD placed fictional Irish religieuse named Fidelma, who is qualified in Brehon law as an advocate.    They are excellent reads. The first one is called Absolution by Murder, but reading them out of order doesn't hurt all that much.