The spider's web : a mystery of ancient Ireland / Peter Tremayne.

Enhanced descriptions from Syndetics:

In the spring of 666 A.D., Sister Fidelma is summoned to the small Irish village of Araglin. An advocate of the Brehon law courts as well as a religieuse, she is to investigate the murder of the local chieftain. While traveling there with her friend Brother Eadulf, a band of brigands attacks the roadside hostel in which they are staying and attempts to burn them out. While Fidelman and Eadulf manage to beat back their attackers, this incident is only the first in a series that troubles them. When they arrive at Araglin, they find out that the chieftain was murdered in the middle of the night, and next to his body, a local deaf-mute man was found holding the bloody knife that killed him.
While everyone else seems convinced that the man's guilt is obvious, sister Fidelma is not so sure. As she investigates, she's convinced that there is something happening in the seemingly quiet town--something that everyone is trying very hard to keep from her. In what may be the most challenging and confusing situation that she has yet faced, Fidelma must somehow uncover the truth behind the chieftain's murderer and find out what is really going on beneath the quiet surface of this rural town.