Van der Flier was the only Ireland player on the field for all 160 minutes of the 2019 and 2023 quarterfinals.
While the All Blacks beat Ireland by 32 points, 46-14, in Japan in 2019, such a loss was easier to accept than the four-point 24-28 defeat in Paris last month.
Ireland was not carrying the form in 2019 that saw it go into the 2023 Cup as a hot favourite, so its non-qualification then was not so disappointing.
Paris had been agonising by comparison. It didn't help that it was only the width of All Blacks' second five-eighth Jordie Barrett's hand under the ball that contributed to the eventual loss.
Van der Flier told the Irish Examiner there would always be regrets about the loss.
"Because it's such a close game, you look back at every mistake you made or something you felt you could have done better, and you're like, 'if I had just done that better, that would have made the difference of this had just gone right or if I'd ran that line...'
"Do you know what I mean? All those kinds of things, which does make it a bit more difficult. So, that was one of the things I found a lot tougher.
"You have to go back to your process and say if you'd done everything you can in the build-up, and you made every effort you could, then are kind of happy with that, but it is hard in reality to do that."
In van der Flier's case, Leinster, the players were given a month off before rejoining their clubs. But he went through 'no man's land' for a while.
"Once the World Cup was over then, once the final had happened, there was no feeling of 'oh, we could be there if it had just gone our way.' It was kind of like just a line in the sand, 'it's over now, let's move on'."
Van der Flier will renew ties with former Tasman and Crusaders coach Andrew Goodman, the Leinster attack coach this year, while World Cup-winning coach Jacques Nienaber will soon take over as head coach.