->''“Find out, though the seas rise to engulf us, or the sky falls to crush us; only the truth is sacred.”''

Series of historical mysteries by British historian Peter Beresford Ellis under the pen name of Peter Tremayne. Sister Fidelma of Cashel is a 7th-century Irish religiuse, educated at the abbey of Kildare. Also sister to King Colgu of Muman (modern Munster), and a trained dalaigh, or lawyer, Fidelma travels Dark-Ages Europe with her friend, [[spoiler: later husband]] the Saxon Brother Eadulf, former gerefa (magistrate) of Seaxmund's Ham (modern Saxmundham, Suffolk).

Ellis uses Fidelma and Eadulf to expound to the reader on the society of Ireland during this period, and on the complex web of cultures, places, religions and rivalries. Irish society was surprisingly FairForItsDay, compared with its neighbors. The stories often focus on specific points of law, or on comparison between societies and religions. [[WriteWhatYouKnow Ellis brings up the following points particularly often]]:
** Position was not strictly hereditary; kings and chiefs were elected by their families, and could be removed from their posts.
** Up until the 9th century or so, many religious houses were coed; priests, monks and nuns could marry, and women could be priests and even bishops. At the time of the series, what we now call Catholicism was starting to gain ascendancy over Celtic Christianity, but it was not yet all-powerful.
** Women's rights were relatively progressive. Women could take significant social roles such as warriors, rulers, and lawyers; they could divorce their husbands and have child custody and alimony; they could own their own property and choose their husbands.
** There were many laws for the safety of citizens and communities. Trials were a right; innkeepers were required to keep clean establishments and lit lanterns; workers in dangerous professions were required to safeguard their workplaces. There were also many types of legal agreements, for example a dozen kinds of marriage.

There are 21 novels and two anthologies of short stories. The first, ''Absolution by Murder,'' was published in 1994.

!!This series provides examples of:

* AlwaysMurder: At least three in every novel.
* ArtifactTitle: Sister Fidelma quits the religious life about halfway through the series.
* AsskickingEqualsAuthority: Irish chieftains were required to be of sound body, and stepped down when physically unfit. In one book, Colgu is attacked but the assassins don't even try to kill him, just injure and disqualify him for the kingship.
* AsYouKnow: Constant, both in-dialogue and out.
* BusmansHoliday: Fidelma can't go anywhere without finding a mystery to solve, much to Eadulf's chagrin.
* ChristianityIsCatholic: Averted every which way; while what will become Catholicism is recognizable, there are undercurrents of conflict both within it (even the Pope, while disapproving, didn't technically outlaw coed houses at the time) and between it and Celtic Christianity (different methods of crossing oneself, different tonsures, different ''official'' attitudes towards women). Other, more exotic varieties of Christianity also appear, such as Ethiopian.
* ExoticDetective: Both to the reader (as a way to introduce a little known time and culture) and within the story; while Fidelma is well-known and accepted within Ireland, her Saxon friend Eadulf is less welcome, and this reverses itself outside Ireland where women are not expected to be so forthright, or to have such education. Or for a princess of Munster to concern herself with the doings of commoners who aren't even Irish.
* HistoricalDomainCharacter: King Colgu, and several other historical kings and priests. The Synod of Whitby is the setting of the first book, and several of its actual participants are important characters.
* PoliticallyActivePrincess: Many of the murders have political implications for Colgu and his kingdom of Muman, so Fidelma often has to be diplomatic.
* SignificantGreenEyedRedhead: Fidelma.
* SummationGathering: Almost every time.
* TakingTheVeil: Played with; religious houses even before Christianity were closer to schools than places of worship. Fidelma becomes a religiuse primarily because of the educational opportunities it offers, and gradually withdraws from the religious life.
* TheWatson: Eadulf. Also, as a foreigner to Ireland, he has ''constant'' exposition spouted at him for the readers' benefit.