An Iron ingot is an item in the Mod, that was originally added in vanilla.
Iron ingots can be most easily obtained by mining and subsequently smelting iron ore, which can be found in every biome, with the exception of Mordor. In addition, it can sometimes be found when slaying a Rohan or Gondor Blacksmith. It can also be bought from dwarf miners or found in chests in ruined houses and many other generated structures.
Iron ingots, or iron ore, are also involved in the production of Dwarven steel, Blue Dwarven steel, Elven steel, and Galvorn ingots. To make these alloys, you'll need an Elven or Dwarven forge and the correct ingredients.
If you've got chests full of iron that you simply want to get rid of, you can always sell it to any type of smith, who will pay you money for each ingot.