The Devil's Seal: A Mystery of Ancient Ireland (Hardcover) by Peter Tremayne

Ireland, A.D. 671. An Anglo-Saxon delegation arrives in Cashel to debate the new religious rules that have been handed down from Rome. The Abbot of Imleach leads the Irish delegation, which is hostile to the new rules from outsiders. Among the Anglo-Saxon group is Brother Eadulf's own younger brother, Egric, whom Eadulf hasn't seen for many years.

When the debate quickly becomes acrimonious, a local abbess has to step in as a mediator between the two sides. But not even a day later her body is discovered, bludgeoned to death. The Chief Brehon Aillín accuses young Egric of murder, and suspicions and tempers run high. With the war of words threatening to spill over into bloodshed, Fidelma is sure there is something more sinister behind the murder than religious differences, and she is resolved to find out what really happened-and why.