Newcomer Lucy (represented by A&J Management) stars as 12-year-old American girl, Mickey, who is destined to release a small Irish town and its people from an ancient curse.
To succeed, Mickey must redeem a ghostly Black Knight from his long quest and single-handedly save a magical herd of horses from a ruthless woman with evil plans.
Lucy previously played the role of 'Flora' in 'Turn of the Screw' at the Almeida Theatre.
The cast also includes Lorcan Cranitch, Thekla Reuten, Fiona Glascott, Sean Mahon and Miriam Margolyes.
The Dutch-Irish-German live-action film is written by Nadadja Kemper, directed by Lisa Mulcah and shooting took place in Wicklow County, Ireland, and Germany last year.
Julia Weber, head of sales at Global Screen, said, " ‘The Legend of Longwood’ has all the ingredients for a hit feature: beautiful horses, a magical world, adventure and a strong female hero to attract the kids, mystic settings, history, aristocracy, weddings and romance for the adults, and fanciful themes like magic, malediction, chivalry and black knights."