The lost island - Eils Dillon

Michael Farrell was forced to grow up quickly after his father
disappeared hunting for treasure on the fabled lost island of
Inishmananan. Struggling to get by, one evening he and his mother
receive a mysterious message from a ragged tramp who stops by their
farm. The old man has proof that Michael's father is alive!

Although no one seeking the island has ever returned, Michael and his
friend Joe board the first boat they can, only to find out it is run by
a treacherous gang of sailors. Braving the unknown seas, they embark in
a grand search for Michael's missing father, the spectacular fortune,
and the island's long-lost secret. Set amid Ireland's picturesque west
coast, plots against Michael and the adventures that befall him make
this magical and suspenseful narrative a page-turning, rough and tumble
adventure story.