The Storm Keeper's Island / Catherine Doyle.

Enhanced descriptions from Syndetics:

An evocative tale of ancient magic, bravery, and family bonds.

"Fans of Harry Potter or Percy Jackson can add Fionn Boyle as a generous and brave hero from the Emerald Isle." - School Library Connection

Fionn Boyle comes from a long line of brave seafarers, people with the ocean behind their eyes. But he can't help but fear the open sea. For years, Fionn's mother has told him stories of Arranmore Island, a strange place that seems to haunt her. Fionn has always wondered about this mysterious island, and from the day he arrives he starts noticing things that can't be explained. He can sense the island all around him, and it feels like the island is watching him, too.

Once in a generation, Arranmore Island chooses a new Storm Keeper to wield its power and keep its magic safe from enemies. The time has come for his grandfather, a secretive and eccentric old man, to step down. But as Fionn and the other descendants of Arranmore's most powerful families fight to become the island's next champion, a more sinister magic is waking up, intent on rekindling a long-ago war and changing Fionn's life and the island's future forever.