Actor-Shared Background: While Robert De Niro is most well known for his Italian-American gangster roles and is himself Italian-American, he actually has more Irish ancestry than Italian, being only a quarter Italian and about 30% Irish (his father was half-Irish, and his mother had some as well). Previously, he had played the Irish-American gangster Jimmy Conway for Scorsese in GoodFellas.
Fake American: Brit Stephen Graham (of Snatch. fame) plays a Italian-American mobster (again) and maintains a impeccable New Jersey accent throughout the film.
Irony as She Is Cast: Jimmy Hoffa is shown to be prejudiced against Italians/Italian-Americans and often hurls anti-Intalian slurs during conversations. He is played by the very Italian-American Al Pacino.
Saved from Development Hell: Martin Scorsese has been thinking about directing the film since about 2007. Netflix eventually ensured it got made. The project was thought of as too costly for its niche genre by the traditional movie studios Scorsese previously tried to sell it to.
The film cost between $159 million and $200 million to make, allegedly due to the extensive use of Digital Deaging technology on several characters in it, and likely the reason why traditional film studios refused to produce it. It is the most expensive film Netflix has produced up to this point (think of the average budget of a solo Marvel Cinematic Universe movie, for comparison). It's not the first project of this size Scorsese has directed however. 2011's Hugo was similarly expensive, but got theatrically-released, and ended up a Box Office Bomb.