Add the granulated sugar, water, key lime juice, lime zest and butter to a medium saucepan. Bring to a boil on high heat, stirring often. Boil for a couple of minutes without stirring. Remove from the heat and season with salt.
Add the popped popcorn to a large bowl and pour the cooked lime syrup over the popcorn. Stir to coat thoroughly. Spread the coated popcorn on a sheet pan lined with parchment paper and allow it to cool.
Add the cooled popcorn to a large serving bowl and toss with the graham cracker crumbs. Melt the white chocolate chips in a bowl set over a pot of simmering water. Drizzle over the popcorn and top with decorative sprinkles and slices of lime, if using.
Store any leftovers in an air-tight container on the counter.