The movie was very loosely based on Herbert Asbury's book of 1927 called The Gangs of New York, which tried to make it seem as though there was a continuous line from the gangs of 19th c. NYC to the Prohibition-era ones ( it's a common enough book in libraries, and a fun read with vivid illustrations). But Asbury did little actual research. Tyler Anbinder, a historian who wrote on the Five Points neighborhood, and who was asked to also review the script, said in a National Geographic interview, "The book The Gangs of New York says there was one tenement where there was a murder a day. At the period of time he was writing about, there was barely a murder a month in all of New York City,". The director obviously paid little attention to him. Though there were nationalist Know Nothings who hated immigrants and were a political force before the Civil War, and there are some nice re-creations of Jakob Riis photos in the film, the whole thing seems to be largely a Scorcese fantasy.

For me one of the strangest things was the soundtrack, which had the very singular Mississippi band of Othar Turner, whose Morning Star Fife and Drum Band carried on a deep south black fife and drum tradition for decades. It's great stuff, but has as much relevance to 19th c. New York as Mariachi ballads.