After you complete dungeon 5, you can go do Dungeon 6: Dragon.
Using the Whistle
The whistle works as follows: It will warp you to any dungeon that you have completed (i.e. you got the Triforce Piece from it). Each time you use the whistle in front of a dungeon, you will warp to either the next or the previous dungeon. If you use it when facing north or east, you go to the next dungeon in the sequence. If you use it when facing west or south, you go to the previous dungeon in the sequence. If you use it more than once before the whirlwind catches you, you will skip that many dungeons in the sequence. Also, if you have completed the dungeons out of order or have skipped any dungeons, the whistle will still go in numerical order, and it will skip any uncompleted dungeons in the sequence.
Go to the Dungeon
First, warp to Dungeon 5 using the whistle (see the previous section for details about how the whistle works). Then go south, west, then keep going west until you see some stairs going south (use the Stepladder to cross the river on the way). Go down those stairs going south. Then go west. If you don't have the Power Bracelet, touch the top-right statue to make it move. Get the Power Bracelet that was under it.
Now go west, south, west, and north. You will be at Dungeon 6: Dragon.
Get a Key
Go east from the entrance and use the Blue Candle to light up the room. Kill the Wizzrobes to make a key appear. Take it.
Get the Compass
Go west twice, then go north. Kill the Zols to make the Compass appear. Take it.
Get a Second Key
Go north. Kill all the Keese to make a key appear and the north door open. Take the key and go north.
Get a Third Key
Carefully go past the spike traps in this room, then go north.
Kill the Wizzrobes and Like-Likes in this room, then push the leftmost block to make the north door open. Go through.
DON'T GO NORTH unless you want to fight Gleeok again. Instead, bomb the middle of the east wall and go through. (You can optionally kill the Wizzrobes before going through the wall if you want a blue rupee, but I don't think it's worth it...)
Use the Blue Candle to light up this room. Quickly grab the key and go north through the locked door.
Get the Map
Kill the Zols and Like-Likes in this room if you want the map.
Optional: Get a Fourth Key
If you don't mind making a detour that involves having to deal with even more Wizzrobes, go east and use the Blue Candle to light up the room, then grab the key, then go west again.
Get the Magic Wand
Go north through the locked door. Kill the Wizzrobes, then push the leftmost block to make stairs appear. Go down the stairs and get the Magic Wand, then go back upstairs.
Get a Fifth Key
Go south until you can't go south anymore. Use the Blue Candle, then kill all the Vires in this room and go east. Kill all the enemies in this room, then push the block and go down the stairs. Follow the path until you are upstairs again.
Go south, use the Blue Candle, then kill all the Vires to get a key.
Go to the Dungeon Boss
Go west, but be careful of the spike traps in the next room. Then go north through the locked door to reach the dungeon boss.
Stay in the doorway and you will be safe from the fireballs that Gohma shoots at you. Gohma will move around, and sometimes it will open its eye. When it does, shoot it in the eye with an arrow. If you hit its eye, it will die. Take the heart container and go north to get your Triforce Piece.