Irony is commonly used in U.S. English to refer to a type of rhetoric whereby a statement or assertion is made with the intention to put across the opposite meaning. E.g. "Oh I really like your coat." In British English this is more commonly referred to as sarcasm.
In British English, irony refers to an action or occurrence that is incongruous or amusingly unexpected. An example of this definition of irony might be a boxer betting his life savings on a fight he expects to lose, and then accidentally winning the fight.
Something made of, or largely composed of, iron can be described as irony.