Masterton, Graham. Katie Maguire series.

Sergeant Kieran O'Regan was responsible for hunting down killers. Now he's become one of the dead: his decapitated body has been found in a graveyard, a tin whistle stuffed into his throat. O'Regan was due to give evidence at a trial for police corruption. His gruesome murder sends a clear message to whistleblowers: only silence is safe. DCI Katie Maguire is determined to uncover the truth. But corruption in the Garda stretches back decades. As more officers are horrifically silenced, Katie must decide who she can trust