Information from http://www.halo3planet.com/halo-3-skull-locations/
Important Tip:
If you cant locate the skull try the level from the beginning. Some skulls wont show up if you start in the middle of the level. ( for me: Iron Skull, Famine Skull, Thunderstorm Skull)
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Iron
Iron Skull
If you die, you restart from the beginning of the level.
Iron Skull Location
The “Iron Skull” is located at the very end of the first level, “Sierra 117″. It is located on the building behind where Sgt. Johnson is being held captive, up on the roof. You can gain access to the roof on the far right side of the building. Travel all the way over to the left side of the roof, and the Skull will be concealed in the corner.
See Video at Bottom Page - Starting 0:18-0:43(Click on image for Larger View)