Changeling's Island by Dave Freer
Tim Ryan is not quite caught shoplifting -- owing to a little help -- but gets in trouble, and finds himself shipped off from Melbourne to an island where his grandmother lives.
The source of his help comes too -- Aed, a spirit of air and darkness -- but it notes that his plane is followed through the waters by a selkie.
And on the island, we have the tale of a city mouse in the countryside, the tale of a boy growing up, and the tale of his heritage and why the selkie came looking for him. It involves beer, a venomous snake, his grandmother's failing sight, catching a sheep, the small school (glad to have one student more), a key, both Irish and Australian folklore, and much more.