Dave Kearney is one of those left behind.
The fine lines that separate the men that are in Japan from those that aren't can come down to something as arbitrary as luck, or the lack of it.
For instance, wind the clock back to that first World Cup warm-up against Italy and Ireland coach Joe Schmidt hands starting berths to Andrew Conway on the right wing and Kearney on the left.
At the best of times, wingers are at the mercy of those who supply them.
Conway touched the ball 13 times, making two clean breaks, beating three defenders to Kearney's seven touches, two breaks and two defenders beaten.
Conway made his two tackles and Kearney made his three.
There was little to separate them statistically, especially with both pinching tries too.
The difference was that Conway's involvements were made with more space to manoeuvre.
"There is just so much luck involved in sport. Games are different. Andy got a lot of ball in that game," recalled Kearney.
"Obviously, I would have liked more game time, more opportunities.
"That Wales away game, I didn't touch the ball once the whole half. That was frustrating because that was my last chance."
Perhaps, it is just as well the younger Kearney is not out in Japan, suffering the slings and arrows that have been fired from all angles at Ireland.
"It's hard to avoid it really, isn't it? You just have to try and not listen to it, try to not read it," he advised.
"For me personally, I don't like going onto Twitter and reading what people have to say, people's opinions or people commenting on performances.
"I try not to do that and I probably learned that from the last World Cup.
"The Argentina game is still, by far, the worst day I've had in my career."
The scar from missing out on the World Cup has healed up quite nicely.
Kearney showed what he could do against Benetton in the first round of the PRO14 League.
He touched the ball 15 times to return with a hat-trick in what well could be a reflection of the different philosophies at Leinster and Ireland.
Either way, the reality is that Ireland prepared Kearney to make an instant impact for his province, not the other way around.