The Iron Forge, the object for which Ironforge is named, is an enormous artifact is a legacy left behind by the Titan Khaz'goroth for the the dwarves' Earthen ancestors. The Iron Forge has never been used by the dwarves, but is kept within the Hall of Thanes (also known as Old Ironforge) to honor the dead kings.
Brann Bronzebeard believes that the Titans left the forge behind for some purpose the dwarves need to find, a purpose which may be uncovered as the dwarves learn more about their ancestry.