Master of Souls: a Mystery of Ancient Ireland (Sister Fidelma Mysteries) 1st edition

In January of 668 A. D., Fidelma of Cashel - sister to the king of Muman, advocate of the Brehon law courts and religieuse in the Celtic Church - is called to investigate the brutal murder of Abbess Faife and the mysterious disappearance of six young female religieuse while away on a short pilgramage away from their abbey.  But when Fidelma and her husband, Eadulf, arrive, they are confronted with another violent murder under mysterious circumstances. One of the senior scholars was bludgeoned to death in the oratory. 
With evidence of shipwreckers and the rumored figure of "The Master of Souls", a mysterious rabble rouser, roaming the nearby countryside wrecking havoc and raising rebellion, both complicating matters, the redoubtable Fidelma is faced with her most perplexing mystery ever.