This book should come with a warning sticker: "Do not open if you want to get anything done". It's the first in a series featuring Dublin Inspector Mike Mulcahy & is compulsive reading.
It has everything you want in a great police procedural. Smart, likeable protagonist, department politics, atmospheric setting & a really scary bad guy.
It's difficult to believe this is a first novel. The prose is clean & smooth, dialogue is taut & the pacing is bang on. About halfway through the identity of the "Priest" becomes evident but even that doesn't diminish the suspense or tension.
If you're a fan of Val McDermid, Stuart Neville or Adrian McKinty, give this a try.