While The Irishman isn’t all that different from some of Martin Scorsese’s other mob-inspired films, at least in structure and story, the movie follows its characters over the course of decades, and so digital technology was used throughout the film to make actors like Robert De Niro appear younger.
The Irishman was always going to be an expensive movie.
It had a cast that included Robert de Nro and Al Pacino, and you gotta figure Martin Scorsese had to make it worth Joe Pesci’s while to come out of retirement, so the salaries alone were going to add up, but the much bigger expense was the digital effects.