Three things I'd like to address up front.
First, the box cover assertion that this is "The best gangster film since Goodfellas." No, actually, it's not. I'm tempted to give that honor to "The Departed," but once again, that wasn't a classic "mob" movie in the sense of The Godfather, Goodfellas or Casino.
While "The Departed" was a movie "about" the Irish mob, the real emphasis of the story was on the law enforcement officials...both upright and corrupt...caught in the vortex of trying to take the bad guys down.
Second, Val Kilmer. He's prominently featured on the box cover, he's got third billing, and his character is basically a generic cop who also narrates the film. The role could have been played by anyone. If you're fresh off of watching "Tombstone" and are thinking "Oh boy, a new Val Kilmer movie," think again. I don't want to give away any spoilers, but his interactions with Ray Stevenson's Danny Greene character are flat and unconvincing. Want to see genuine interplay between characters who grew up in "the old neighborhood" and are now damaged adults? Watch Sean Penn, Kevin Bacon, and Tim Robbins in "Mystic River."
Third, Christopher Walken. This isn't "a Christopher Walken movie" any more than it is "a Val Kilmer movie," but he's in it, and that means expectations are going to be high among viewers with even a passing interest in his work. He's dead-center in the box cover artwork. He's not dead-center in the movie. As mobster Shondor Birns, he represents a turning point of sorts in Greene's life. Freshly ousted as head of the union, Birns offers him a "collections" position in his loan sharking business. Once again, I don't want to offer up a spoiler here, but the business relationship goes sideways and Greene's method of handling it takes his journey to another level. What you don't get, in the highly compressed time frame of a 106 minute movie, is the dynamic of the relationship between Birns and Greene...it is almost as if you are expected to assume that Birns is the bad guy because he is portrayed by Walken, who tosses off a few Walken-isms along the way. When the moment of truth arrives, you aren't left with a deep understanding of how things got to that point. You've been offered the events, but there's no glue to hold them together.
Having said all of that, this might have made a far more engaging HBO (it would have to be on cable, due to the language and graphic violence) mini-series than a stand-alone movie, but it's still a 4-star movie.
There is an excellent one hour documentary included, "Danny Greene: The Rise and Fall of the Irishman," which I decided to watch before the movie...and I was glad I did. While it is a spoiler of sorts (you know the beginning, middle and end of the story before you watch the movie), when you actually do watch the film, there is a sense of cohesiveness I don't believe would have been there without it.
It's just too big of a story to fit into one movie. Greene was the epitome of the enigmatic "Robin Hood" type, and as his first wife shares in the documentary, he could display great acts of kindness (10 paid scholarships in the best private school in town for 10 inner-city kids from poor families, 50+ hams at Easter to neighbors, 50+ turkeys on Thanksgiving and Christmas to the same neighbors, paying 3 months' back rent for a neighbor, ridding the neighborhood of a rowdy biker gang), he was also prone to acts of extreme violence.
I can't predict how you'll react to the good man / bad man balance in the Greene character. I'm also not a scholar when it comes to the real-life events so I can't tell you if he is portrayed as a "better" or "worse" man than the real-life Greene.
Within 5 years of his death, as the result of a chain of events that followed, the FBI had essentially wiped out the U.S. presence of La Cosa Nostra. So it is impossible to separate "good" and "evil" in this movie...and it is usually the same in real life. Good people do bad things. Bad people do good things. There is no black and white sharp defining line. We blur those lines every day that we are breathing.
The movie also effectively incorporates news footage of the real Danny Greene and news reports from the era. The story is brutal. When we see Greene sitting with his new girlfriend and discussing a move to Texas, to get away from it all and start a new life, it's pretty much a guarantee that he's not going to make it to Texas.
So if you set your expectations to what I have shared here...that this is not "the best gangster film since Goodfellas," that the performances by Walken and Kilmer are far from "career defining," and that you will need a sense of history and the chain of events to understand how things get from point "A" to point "B," and then to point "C"...I think you'll really enjoy this film.
Most likely, if you watch the film and documentary in whatever order you choose, then watch the film a second time, you will enjoy it more than if you just hit the film cold on your first viewing.