Where are some possible locations to use the magic mirror so I can get back to the Dark World? I used the mirror while in the Dark World, then left the spot to get some heart containers. Now I can't get back to that spot(its on an island). Are there spots in the world I can just use at any time? Any help would be appreciated

You can only use the magic mirror to return to the light world. Using the mirror leaves a portal to the dark world. If you cannot reach that portal, there are around 8 other permanent portals around the light world that lead back to the dark world. They look like little square pools of water.

Finally, you can get back to the dark world by walking though the big open gate that leads to hyrule castle.