The Hounds Of The Morrigan by Pat O'Shea

Two children stumble into a battle between the modern world and ancient Irish magic when an old, dangerous mythology reawakens; pursued by supernatural hounds and the fearsome goddess behind them, they embark on a urgent journey through a landscape where folklore and reality blur, drawing on old stories, unlikely allies and their own courage to outwit shapeshifting foes and to put myth back in its place before catastrophe can be unleashed.

The 8009th greatest book of all time