With a book and lyrics by Lynn Ahrens and music by Stephen Flaherty, ONCE ON THIS ISLAND, based on the 1985 novel My Love, My Love, or the Peasant Girl by Rosa Guy, a creative retelling of Hans Christian Andersen’s “The Little Mermaid,” takes place on a Caribbean island divided into two worlds: the peasants and the rich Grand Hommes. These worlds never cross paths until, one day, Ti Moune, a pe…