Island of the Great Yellow Ox (1971)

When a terrifying storm washes Conor, his little brother Babo and their two American friends, Mary and George up on to Ox Island off the coast of Ireland, they become prisoners of the eccentric Lady Agnes and the Captain. This pair of scallywags are determined to uncover the Druid treasure they know is hidden somewhere on the island and these unwelcome children will not be allowed to get in the way.