Night of the Lightbringer: Peter Tremayne

Ireland, AD 671. On the eve of the pagan feast of Samhain, Brother Edulf and the warrior, Aidan, discover a man murdered in an unlit pyre in the heart of Cashel. He has been dressed in the robes of a religieux and killed by the ritualistic 'three deaths'.
When a strange woman known as Brancheo appears in a raven-feather cloak foretelling of ancient gods returning to exact revenge upon the mortal world, she is quickly branded a suspect.

But in their search for the killer, Sister Fidelma and Eadulf will soon discover a darker shadow looming over the fortress. For their investigation is linked to a book stolen from the Papal Secret Archives which could destroy the New Faith in the Five Kingdoms . . . and Fidelma herself will come up against mortal danger before the case is unravelled.