The Iron Hills are a range of mountains in the north of Middle Earth, to the east of the Lone Mountain Erebor and to the north of the land of Rhun. It was populated almost totally by dwarves.
After the destruction of Erebor by Smaug, a part of Durin's folk who fled the mountain set up residency in the Iron Hills. After the events of The Hobbit, in which the king of Durin's folk, Thorin Oakenshield died, the ruler of the Iron Hills dwarves, Dain II, moved back into Erebor and became king there. Presumably most, but probably not all, of the Iron Hills dwarves followed him.