To simulate Bill the Butcher's fake eye, Sir Daniel Day-Lewis had his own eyeball covered in prosthetic glass. Day-Lewis learned to tap his fake eye with the tip of a knife without blinking.
Most of the gangs mentioned by name were real 19th-century New York gangs. Bill "The Butcher" Cutting is based largely on real-life New York gang leader William Poole, who also was known as "The Butcher" and had much the same prestige as Sir Daniel Day-Lewis' character.
Leonardo DiCaprio accidentally broke Sir Daniel Day-Lewis' nose while filming a fight scene. Day-Lewis continued to film the scene despite the injury.
Sir Daniel Day-Lewis became so uncomfortable with the greasy hairstyle he wore as Bill the Butcher that he shaved his head immediately after filming completed.
Before the battle at the beginning of the film, several gangs introduce themselves. The Dead Rabbits, the Bowery Boys, and the Forty Thieves were real life New York gangs in the Five Points in the 1860s and 1870's. Their appearance, weapons of choice, and behavior are accurate. Many members became politicians later on.
Martin Scorsese: the wealthy man Jenny "turtledoves" at the head of the table. He also voices one of the telegraphists. He is the one that says "Find the military and send them..." and "18: The mob is very wild".