Rinse thoroughly then fill with a bleach/water solution for a few hours. Then rinse thoroughly again and set the barrel in the sun for a day with the cap off.

Household bleach breaks down very fast in sunlight to salt and water. The solution you buy for laundry is already weak, and when you mix a quart with 55 gallons, there's not much left.

Since the barrel only had syrup in it, it's probably safe to use if you just rinsed it real well.