Without getting too far into plot details, the movie skips around between about four or five time periods in Frank Sheeran's life, each one knocking a few years off of De Niro's face (and, at times, his body). At first, it is weird! Mostly because we know what Robert De Niro looked like when he was in his forties and fifties, and it's definitely not like this. But, truly, after a few minutes you get used to it, probably because every scene lets you get familiar with whatever version of De Niro's (or Pesci's, or Pacino's) face you're looking at. The main thing that looks distracting throughout the whole thing is De Niro's eye color, which was changed to a bright blue -- because that's the eye color Sheeran had.
At a Q&A following the screening, Scorsese, cast and crew recalled the loooong road they took to making this movie, which first started its gears turning all the way back in 2007. However, like James Cameron's Avatar, another example of an effects-heavy film needing to wait years for the technology of the day to catch up with it, The Irishman benefited from all the time left to simmer. "It was slow in moving but not in a negative way," said Scorsese. Producer Jane Rosenthal agreed: "The technology did not slow us down. The technology kept evolving and kept changing and kept making things simpler."
Eventually, they tried a side-by-side test copying a scene from Goodfellas, during the filming of which De Niro had been 47. The test fooled Al Pacino, who remembered thinking that De Niro was just so good he could convince anyone he was 30 years younger. "Wow," he remembered thinking, "he's Meryl Streep!"
You can't just make an old guy's face look young and call it a day, though. The actors also have to act like they're younger than they are, which is tough when you're a 70-something playing a 40-something. "It isn't just about lenses and computer imagery," Scorsese explained. "It's about posture, it's about movement, it's about clarity of the eyes, everything." He recalled a day on set when Pacino, during a particular rant, hops out of his chair in the heat of the moment, and after a few takes stuntman Gary Tacon whispered to Scorsese that Pacino was "supposed to be 49." The next take they did, he asked Tacon how it felt, and he said, "62." Scorsese said, "No, no, no. We gotta get it down to 49." Everyone old is young again.