In a saucepan, over medium heat, melt the butter. Add the ginger and cook, stirring, for 2 minutes. Add the sugar, orange juice, and cranberries. Cook until the cranberries burst and the sauce thickens, about 15 minutes. Stir in the marmalade to taste (depending on desired sweetness). Turn off heat. Stir in the pepper and cilantro just before serving. Serve warm or at room temperature.
Make Ahead: Prepare the relish up to 5 days in advance, omitting the pepper and cilantro. Cover and refrigerate. Warm it to room temperature, then stir in the pepper and cilantro before serving.