Warren Gatland says telling players they will miss out on the 2019 World Cup is the hardest part of his job as Wales coach.
He will name his 31-man squad on Sunday - 24 hours after Wales play Ireland in a warm-up match in Cardiff.
Gatland says seven or eight places are still up for grabs before he announces his final party for Japan.
"Sometimes you get one chance and for some of them that chance is on Saturday," said Gatland.
In his final home match in charge of Wales before he quits after the 2019 World Cup, Gatland has made 14 changes from the side that defeated England.
It is a second-string Wales side but a team that will be having a final audition for a World Cup place.
When asked which players he knew would be on the plane to Japan, Gatland said: "I haven't gone through that exercise definitely, but probably 23 or 24 I suppose.
"Having spoken to the players they are aware of the opportunity, they have made a lot of progress in the 12 or 13 weeks together.
"The training camps have been fantastic and we've been very impressed with the players."
Following the World Cup selection meeting on Saturday night, there will be nine disappointed players missing out when the party is announced at 14:00 BST on Sunday.
The players have been given different options on hearing the news ranging from a telephone call, email, text or finding out live.
"It is the hardest part of the job," said Gatland.
"It's tough because I know first-hand what it's like to miss out on a World Cup, because it happened to me with the All Blacks in 1991.
"I wasn't selected in the squad and was watching it live on TV. I know how disappointed those players who aren't selected are going to be when they watch the squad on Sunday."
Ireland coach Joe Schmidt has made 11 changes to the team that were heavily beaten 57-15 at Twickenham last weekend.
Wales will retain their world number one ranking if they win on Saturday, but will drop to second if they draw and fourth if they lose.
In turn, Ireland will take top spot if they win by more than 15 points in Cardiff.
Wales: Amos; Lane, S Williams, Watkin, S Evans; J Evans, A Davies; Carre, Elias, Lee, Beard, B Davies, Shingler, J Davies Navidi (c).
Replacements: Dee, R Evans, Brown, Ball, Moriarty, T Williams, Patchell, Holmes.
Ireland: Addison; Conway, Farrell, Aki, Stockdale; Carty, Marmion; Kilcoyne, Scannell, John Ryan, Henderson, James Ryan; Beirne, O'Mahony (c), Conan.
Replacements: Best, Porter, Furlong, Toner, Murphy, L McGrath, Ringrose, D Kearney.
Referee: Romain Poite (France)
Assistant referees: Jérôme Garcès (France), Karl Dickson (England).
TMO: Rowan Kitt (England).