That is the island of Driftwood I do believe. A series of side quests involve it; you can earn the Island Paradise achievement by completing them. The area is accessed from Millfields, but only after you give a man 750 gold to repair a bridge in the Restoration side quest.
If you come back later to the area, the bridge is fixed and you can cross it into the Driftwood area. The island will not immediately be marked on the sanctuary map so you can fast travel to it. You'll have to finish the series of quests to establish the island of Driftwood by helping the future inhabitants. Until then, you'll have to go through Millfields.
Getting to the bridge, or the area where the man will be waiting:
Fast travel to Millfields. You'll follow the main paved path for quite a while. Don't turn off the road and you'll be fine. You'll spawn next to fields, pass the area with all the miners and the monorail, go through woods, and pass into a more grassy area. You maay see a noblewoman or two along the way. Shortly before you get to the place you're looking for, you'll walk over a bridge with decorated stone railing. Right away you'll reach a "V" intersection - take the left side. A group of balverines almost always spawn there. Follow and take a right, and once the bridge is restored, it will be there: a brown, wooden one.
I've included a picture of the intersection, with the area where the bridge to Driftwood will be built underlined in red.
After you gain access to Driftwood, walk through the area to the beach, where a man will ask you to exterminate some hobbes. After reaching him, you'll be updated about quests you'll need to complete to establish Driftwood island - some time will pass between when they're assigned.
I hope you have found all the information you need! Good luck, and post more questions if you have them.
*The "landmarks" I've mentioned may vary, depending on the point you are at in the story. However, the V intersection at the lamp post will definitely remain the same, and most else will.
