The Iron Hills were a range of hills in the northeast of the known parts of Middle-earth, border-lining Rhûn. There was also a Dwarf-realm located in the hills. The Hills were some of the remnants of the Iron Mountains where Morgoth dwelt, and had a heavy iron deposit, from which they get their name
The Hills were mined for thousands of years by by the Dwarves, but officially became a Dwarf-realm in T.A. 2590, when Grór son of King Dáin I was exiled from the Grey Mountains by the Dragons. Over hundreds of years it became a great realm, especially after the destruction of Erebor. Before the death of Smaug, the Iron Hills was the main dwelling of Durin's folk in the East, and was also the strongest realm militarily in Wilderland.