The mercurial 26-year-old, capable of playing fly-half, centre and full-back, is widely recognised as one of the most talented Welsh players of his generation but has failed to make the first XV despite recovering from a shoulder injury.
Leigh Halfpenny takes over at full-back while Rhys Priestland starts at fly-half and Jamie Roberts is paired with Jonathan Davies in midfield.
And Gatland, who has also left British and Irish Lions fly-half Stephen Jones out of the match day 22 entirely, admits it was tough call to leave the pair on the sidelines.
"This was a tough selection for us, probably one of the longest selection meetings we have ever had where we went through every possible permutation available to us," said Gatland.
"It's a great position to be in when you are struggling to find a place for players in the squad and the starting line-up, instead of having to think who can make up the numbers, which has sometimes been the case.
"This is rugby World Cup quarter-final time and we had some tough decisions to make with the likes of Dan, Shane and James returning from injury and the side having done so well in their recent absence.
"But this is the side we have gone with and, as a 22, we believe it is the side which gives us the best chance of winning this game against an Ireland team which, not only topped their group, but beat the Wallabies on the way."
Dan Lydiate also returns to the side in place of Ryan Jones, while Shane Williams has recovered from injury to take his place on the wing and Alun Wyn Jones is back at lock.
Wales team to face Ireland:
15. Leigh Halfpenny - Cardiff Blues
14. George North - Scarlets
13. Jonathan Davies - Scarlets
12. Jamie Roberts - Cardiff Blues
11. Shane Williams - Ospreys
10. Rhys Priestland - Scarlets
9. Mike Phillips - Bayonne
1. Gethin Jenkins - Cardiff Blues
2. Huw Bennett - Ospreys
3. Adam Jones - Ospreys
4. Luke Charteris - Dragons
5. Alun Wyn Jones - Ospreys
6. Dan Lydiate - Dragons
7. Sam Warburton - Cardiff Blues
8. Toby Faletau - Dragons
16. Lloyd Burns - Dragons
17. Paul James - Ospreys
18. Bradley Davies - Cardiff Blues
19. Ryan Jones - Ospreys
20. Lloyd Williams - Cardiff Blues
21. James Hook - Perpignan
22. Scott Williams - Scarlets