Keane came into Andy Farrell's camp before they lost to England at Twickenham and, while his words didn’t have the desired effect on the Saturday, they had a lasting impact on the skipper who is a life-long United fan.
Sexton, who hopes to return from injury for Leinster's away trip to Montpellier on Friday week, was speaking at the launch of the Heineken Champions Cup when he was asked about Keane’s chat with England captain Owen Farrell last year.
"We had Roy Keane with us ourselves probably 18 months ago before we played England at Twickenham. We lost, so he didn't have that big of an impact on us, but it was a brilliant couple of hours with him," Sexton said.
"He was incredibly generous with his time and his stories, but I was actually delegated to ask him questions on the night. I asked him one question and he spoke for two hours, so I was doing my best job, writing questions all week.
"I was researching and searching different avenues that I could probe him on and I asked one question and lads slagged the arse off me. But yeah, it was very special from that point of view.
"It was probably very surreal sitting there listening to him. He was very, very open with us. We asked him about different things about leadership, and about team culture and environments. We probably didn't get the answers we thought we were going to get."
Now in his third season as Ireland captain, Sexton took little nuggets of Keane's wisdom into his own day to day rugby life.
"What he boiled down to was: effort, hard work, turning up day in, day out. He gave us very simple answers, but brilliant answers all the same," he said.
"You sometimes get speakers in and they've got buzzwords and they talk about lots, but with him, it was just getting to the point. He wasn't mad about all the talk about leadership groups and all this stuff, he was more saying that when he was at (Manchester) United, he just had good people around him and that drove standards. And they probably didn't even know they were doing it. But it was a brilliant couple of hours.
"We could have sat there all night listening to him. And it's the same when you watch him on TV now: you can't turn him off when he's on the screen."
Normally, players are their own worst critics and then there's the media.
It turns out Sexton has another reviewer back at home, as his eldest son Luca has taken to dissecting his performances as he grows more interested in daddy's day job.
"They are (excited) now, but Luca didn't at the start. He used to come to the games and Laura (Sexton's wife) used to have to bring an iPad for him to watch stuff on and I think over COVID times he has grown up a lot and he's taken a strong interest in sport," he explained.
"He supports Liverpool, which is breaking my heart because all his mates support Liverpool. His favourite player is James Lowe, which also breaks my heart.
"But it's incredible. The honesty that they come out with is brilliant as well. They sometimes say the things that you don't want them to say.
"After the All Blacks game, he said to me: "Dad, you missed two, and your replacement got three." I've just gone: "Ah, it's started already." So, it's got some challenges as well."
Although he picked up ankle and knee injuries in the win over New Zealand, Sexton's performances in his 100th and 101st caps in November suggested he remains right at the top of his game.
"I don't like talking about myself too much, I'll leave that up to other people, but I feel good, I feel good on the pitch," he said of his form.
"Often - I'm not being critical - you guys might look at kicks or tackles made. Often some of the things you're proudest of or most pleased with go unnoticed.
"It's the small things. I think the thing for me is, as an out-half, you've got to get the team playing well, you've got to get the team flowing and you've got to get the team in a good place, and when the team does that, that's what I take the most satisfaction from.
"Sometimes, you can have moments in a game where people go: "Wow!" and other times you don't.
"But the times when you don't have special moments are sometimes your best games. I suppose the best thing I can say is that we played well in the two games I was involved in and that's what I'm happiest about."
On Saturday, Leinster will kick off their European campaign against Bath and it looks like Sexton will have to lead from outside the starting side as he gets his body right for the battles ahead.
"It's important that I'm there for the team at all stages. If I'm injured, I'm there to help, and if someone asks questions, of course, I'll be as open as I can, but at the same time, there are guys there that are playing out-half and there are guys there leading the team from the captain that need to do it their own way, and they don't need me in there overbearing them," he said.
"So, it's a balance, isn't it? It's being there for them, making sure they're okay and we're preparing well, and if you see something maybe having a quiet word to one of the coaches or with the captain or the other out-half who's playing. But I hope to be fit, hopefully in Round 2.
"So, that's the plan, that's what I'm working for. I think the Bath game will probably be a bit too soon for me, but if I can get back for the Montpellier game, that's the goal at the moment."