To unlock the Iron Keep Bonfire achievement you must light the bonfire located just after the Old Iron King boss fight. The Old Iron King is the builder of the Iron Keep and is a large demon that dwells in the sea of lava that surrounds the castle. His fog door is located out on the lava sea, just after the third bonfire in the Iron Keep.
The King is vulnerable to lightning-based attacks, including lightning infused weapons and the Lightning Spear miracle available for purchase from Licia of Lindeldt. You can also find a copy of the Lightning Spear miracle in the Earthen Peak, near where you encounter Mild Mannered Pate. I went ahead and attuned both, given that my Faith at this point in my playthrough was 23+.
The Old Iron King doesn't move very much; he'll stand in the lava and use two styles of attack against you. If you're too close, he'll use his arms to sweep and slam his fists against the stone platform, which does lots of damage. If you're further away, he will employ various fire-based attacks, including: launching various beams of fire from his mouth, breathing a sweeping string of fireballs across the arena, and using a double-fisted slam to trigger a Firestorm.
For magic users, focus on using Lightning Spear from a distance. There's a bit of wall on the far right side of the arena you can try to use for cover, but the King's attacks can hurt you through the wall, so be careful. For physical attackers, try to dodge his fist slams and attack his arms as he recovers.
Once the Old Iron King is dead, you'll be able to enter a door behind you on the far right side of the ruin island you fought on. A treasure chest and some stairs await and, at the bottom of the steps, is the Iron Keep Primal bonfire. Light it and the achievement is yours.
Credit for the above image goes to the Dark Souls 2 Wiki (http://darksouls2.wikidot.com/old-iron-king
). Credit for the video below goes to Fextralife (http://www.youtube.com/user/Fextralife