Dogwort Plains
Dogwort Plains is located on Storybook Island. You'll be well into the game before you're able to visit here, and will need a ship to reach the island. The troll bridge is in the southwestern region of the plains. Your characters won't be able to get past him for a while -- they'll need the North Wind to move him, and they can't capture it yet. While your party is in the area, make sure they speak to Doyle in Happily Ever After, a city on the east of Dogwood Plains. Otherwise they have no reason to bother passing the troll.
The North Wind
Retrieving the North Wind is a separate side quest, but necessary if you want to pass the troll. When you travel to Mount Siren, you'll find a wind observatory at the very top. Here, a man named Obert asks you to retrieve the missing North Wind, which is in White Pass. Traveling to White Pass, on the Mainland, won't take you too long because you'll have a dragon by this point, but you will have to actually fight the North Wind before you can capture it. The North Wind is very difficult, as are all the monsters you'll face in the White Pass region, so make sure your characters have high levels before attempting this.
Deciding Between Two Quests
The North Wind is necessary for the completion of two separate side quests, but you have to choose only one of the two to complete. If you bring the North Wind back to Obert at the observatory, he'll give you the North Wind spell, which Gavin can use. But if you want to use the North Wind to remove the troll, you won't get this spell. Before returning with the North Wind to the troll bridge, decide for sure that you don't want the spell and save the game before you act on the choice in case you change your mind.
Reward
The troll on the bridge is guarding Doyle's missing tailor kit. You will have triggered this side quest when you spoke to him in Happily Ever After. If you collect the kit and bring it to Doyle, he will reward you with your choice of either smelly shoes or magic shoes. The smelly shoes, when equipped, make all monsters except boss monsters disappear and the magic shoes give your party protection against spell damage.