Lukhan Tui has earned his first Wallabies start, named at blindside flanker for the series-deciding June Test.
The lineout has been problematic for the Wallabies in the first two Tests, and Tui will bring another option in that set piece, though Cheika said that was only a bonus of his selection.
Reds no. 8 Caleb Timu has dropped out of the team, while David Pocock moves to the back of the scrum for the clash.
"That is obviously an advantage, he's a better lineout operator because he's a taller guy obviously," Cheika said.
"So, I just think he's impressed - he didn't have much of a lead-in as far as his own form because he was off injured and he's come in and he's really impressed, not just in the games, also at training as well.
"I think it's a good reward for a guy who's worked hard throughout the three matches to get a start."
Ned Hanigan comes onto the bench, replacing Tui, after overcoming a medial ligament injury suffered in Super Rugby.
Nick Phipps will start at halfback in place of the injured Will Genia (arm), with Joe Powell earning his first Test spot since November, named on the bench.
The Wallabies and Ireland head into the third Test with one win apiece in the June Series.
Cheika said there wouldn't be any danger of his side shrinking into its shell in the series decider, looking to run the ball more than they've been able to in the opening two matches.
"The way I go, it's usually the opposite way, I think we'll be less conservative," he said.
"You can't win finals without scoring tries and playing footy. I'm a true believe in that.
"They did well last week to restrict us from doing that and we'll be looking to try and open up a bit this weekend for sure.
"I think we have in all the games, (some) with a bit more success than others but the last thing you do when the finals come is shrink. You want to go out there and play."
The Wallabies take on Ireland in the third Sydney Test on Saturday June 23, kicking off at 8:05pm AEST, LIVE on FOX SPORTS, Channel Ten and via RUGBY.com.au RADIO.
IN: Ned Hanigan, Joe Powell
OUT: Caleb Timu, Will Genia (arm)
Wallabies to face Ireland
1. Scott Sio (45 Tests)
2. Brandon Paenga-Amosa (2 Tests)
3. Sekope Kepu (93 Tests)
4. Izack Rodda (6 Tests)
5. Adam Coleman (22 Tests)
6. Lukhan Tui (6 Tests)
7. Michael Hooper (c) (81 Tests)
8. David Pocock (68 Tests)
9. Nick Phipps (63 Tests)
10. Bernard Foley (57 Tests)
11. Marika Koroibete (10 Tests)
12. Kurtley Beale (73 Tests)
13. Samu Kerevi (20 Tests)
14. Dane Haylett-Petty (20 Tests)
15. Israel Folau (64 Tests)
16. Tolu Latu (6 Tests)
17. Tom Robertson (20 Tests)
18. Taniela Tupou (3 Tests)
19. Rob Simmons (84 Tests)
20. Ned Hanigan (12 Tests)
21. Pete Samu (2 Tests)
22. Joe Powell (3 Tests)
23. Reece Hodge (26 Tests)