I liked the idea behind The Irishman, and while the film was slow, I appreciated what it was trying to do, even if it dragged on a bit too long at some points.
However, the main gimmick, the de-aging technology or whatever it is called, was a miss and detrimental to the film.
It reminded me of the Polar Express in a sense that it tried to popularize a new technology, but only managed to cause a huge whiplash among the audience taken aback by the uncanny valley effect.
It was painfully obvious that beneath the fresh coat of paint where old men who were moving and acting like old men usually do. I couldn't bring myself to enjoy the film fully.
Having another actor play the part of younger De Niro would have been better. Or doing the reverse thing - having a younger actor play the whole film, performing the role of the older character later on under the ton of quality make up. For instance True Detective Season 3 did that pretty well.
This way you got a long-winded drama that had too little meat on its bones which was somehow also a tech demo.