The Ghost of Grania O'Malley by Michael Morpurgo

People on Clare Island say that the Big Hill is haunted by fearsome pirates who once terrorized Ireland's coasts. Jessie senses that the Hill possess a magic special enough to give her the strength to deal with a crippling disease. That magic takes physical shape when the ghost of pirate queen Grania O'Malley appears to Jessie with a mission--to save the Hill from a greedy prospector.