The ironhead was a Harley-Davidson motorcycle engine, so named because of the composition of the cylinder heads. The engine is a two-cylinder, two valves per cylinder, pushrod V-twin. It was produced from 1957 until 1985 and was replaced by the Evolution engine in 1986.
This name may also be applied to the Harley-Davidson Sportster motorcycles that used this engine.
References
Harley-Davidson Ironhead engine Wikipedia(Text) CC BY-SA