You can get to Quan Lan bey driving to Halong, taking the ferry to Hong Gai, going to a different port and taking a local boat to Quan Lan (35,000 dong each way). It takes 3.5 hours and currently leaves around 1PM every day. Be careful, if you want to leave on your own schedule, you can have your own boat for around $80 round trip (1 hour or so to go) or if you need to get off the island in a hurry due to injury, even ATI will jack their prices to $80 or even $120 to leave.
Hire a 3-wheeled taxi (pictured) or bring your own motorbike to Quan Lan. Don't rent one, and drive at your own risk. There's very little trafic on the island, though the road conditions are quie poor.
I rented one, it had terrible brakes and unfortunately one of the main roads had a section collapse (also pictured). A number of people had serious injuries that day, and I escaped with second-degree tailpipr burns.
Be careful, there are no medical facilities on the island.