Normally, the only way to reach Turtle Rock was by returning to Lake Hylia to use the portals there. But with the help of a Fire Rod Boost performed south of Dark Palace, it's possible to skip the Turtle Rock portals.
Some of Lorule's overworld is actually connected as one big map but each section is separated by some means. One of these connected maps is Dark Palace and Turtle Rock.
South of the Dark Maze section is a ledge that can be Fire Rod Boosted off of to get onto the rock wall that separates Turtle Rock and Dark Palace. If you walk along the ledge you can eventually drop down into the water below to enter Turtle Rock.