On Thanksgiving weekend, I watched "The Irishman," and I also saw the behind the scene.
Robert Deniro looks a lot younger in the film.
I thought it was another actor who plays his younger version.
And I realized it's the CGI ( Computer-generated imagery ) from director Martin Scorsese's idea. He heard people said to him in Taiwan when he directed another film, "Silence" ( 2016)
Scorsese didn't like the idea in the past, and the CG technician makes the marks smaller on the actors' faces, so it's easier to make the change of the face.
It's the after effect to make Robert Deniro plays the whole movie without makeup and find a younger actor to play his part.