Lightly oil mold; prepare an ice water bath in a large bowl.
In a large saucepan, bring cranberries, sugar, and 3 cups water to a boil, then reduce heat and simmer, stirring occasionally, until cranberries burst and are completely softened, about 10 minutes.
Pour cranberries through a fine sieve, set over a bowl, pressing to release as much juice as possible (you should get about 4 cups, if not add water to equal 4 cups).
Skim off any foam from the surface; stir in maple syrup.
Transfer 1 cup of the liquid to a clean saucepan and bring to a simmer.
Meanwhile, in a small bowl, sprinkle gelatin over 1/2 cup water; let soften 1 minute.
Stir gelatin mixture into hot cranberry liquid in the saucepan; mix gelatin mixture into cranberry maple syrup mixture and stir well to completely combine.
Set bowl into prepared ice water bath, stirring occasionally, to cool.
After about 15 minutes, it should be beginning to thicken, then pour into oiled mold, and refrigerate until set, at least 8 hours, but can be made up to 2 days in advance.
To unmold, dip the top of the mold into a bowl of hot water and run a thin knife around the edge of the mold to loosen the jelly; place the serving dish on top and invert to plate (if it doesn't release easily, repeat).