First off "The Irishman" isn't your standard "mafia" movie so dont go into the film thinking you're going to experience "casino", "goodfellas", "The god father", again...
This film is very much focused around acting, drama, long conversations and the ability to make you connect with the characters.
The story focuses around a hitman, and his journey from meat delivery guy, to the top ranks of an organised crime family, and another strong personal/professional relationship with a man in the political game, who also has ties to organized crime.
At 3.5 hours long It's definitely a lot of story to take in, and a lot to keep up with - but its worth it.
The CGI used to make the main cast look younger is incurable, and you forget its even CGI after a few minutes