After the Silence by Louise O'Neill

Nessa Crowley's murderer has been protected by silence for ten years.
Until a team of documentary makers decide to find out the truth.
On the day of Henry and Keelin Kinsella's wild party at their big house a violent storm engulfed the island of Inisrun, cutting it off from the mainland. When morning broke Nessa Crowley's lifeless body lay in the garden, her last breath silenced by the music and the thunder.
The killer couldn't have escaped Inisrun, but no-one was charged with the murder. The mystery that surrounded the death of Nessa remained hidden. But the islanders knew who to blame for the crime that changed them forever.
Ten years later a documentary crew arrives, there to lift the lid off the Kinsella's carefully constructed lives, determined to find evidence that will prove Henry's guilt and Keelin's complicity in the murder of beautiful Nessa.