The Lions management have made five changes to the team that lost the first Test 26-21 in Durban, with Kearney taking over from the injured Lee Byrne at full-back and Fitzgerald preferred to Ugo Monye on the left wing.
That means Kearney and Fitzgerald will form an all-Irish back-three along with Tommy Bowe who, along with Monye, is the 2009 tour's joint-top try scorer (four).
Ireland supply seven players to the side, which will be their biggest representation for 62 Lions Tests stretching back to the third Test in South Africa in 1938.
On that occasion there were eight Irishmen in a side that beat the Springboks 21-16 in Cape Town.
Notably, the Lions will line out with an all-Welsh front row on Saturday as hooker Rees and tighthead prop Jones link up with their compatriot Gethin Jenkins at loosehead.
The centre partnership of Brian O'Driscoll and Jamie Roberts that did quite an amount of damage in the first Test has been retained, as has the all-Welsh half-back combination of Stephen Jones and Mike Phillips.
Adding his bulk to the second row, 1997 tourist Shaw will scrum down alongside tour captain Paul O'Connell and McGeechan and company have kept faith with the Anglo-Irish back row of Tom Croft, who touched down twice last time out, David Wallace and Jamie Heaslip.
In addition, there have been four alterations to the Lions bench. Hooker Ross Ford, prop Andrew Sheridan and winger Shane Williams are new inclusions in the Test 22 and Alun Wyn Jones has dropped down from the starting line-up to take the place of Donncha O'Callaghan.
Commenting on the team selection, head coach McGeechan said: "As a squad the Lions still have everything to play for.
"The players know that the outcome of the Test series comes down to Saturday and they are determined to take the series to a decider in Johannesburg.
"There were many positives to come out of the First Test which we have built upon this week.
"We have made five changes to the starting side but it will not change the way we want to play.
"Both teams will be better for the run last weekend and we know we will have to step up our performance to ensure that we again take the game to the opposition and are more accurate in taking our opportunities.
"The selected team is an interesting one in that we have an all-Welsh front row for an official Lions Test match for the first time since 1955.
"We also have an all-Irish back three and I am also delighted for Simon Shaw who will play in first Lions Test after three tours and 16 matches in the famous red shirt."
BRITISH & IRISH LIONS TEAM/REPLACEMENTS (v South Africa, 2009 Lions Tour Second Test, Saturday, June 27, Loftus Versfeld, kick-off 3pm local time/2pm Irish time):
15. Rob Kearney (Leinster/Ireland)
14. Tommy Bowe (Ospreys/Ireland)
12. Jamie Roberts (Cardiff Blues/Wales)
11. Luke Fitzgerald (Leinster/Ireland)
10. Stephen Jones (Scarlets/Wales)
9. Mike Phillips (Ospreys/Wales)
1. Gethin Jenkins (Cardiff Blues/Wales)
2. Matthew Rees (Scarlets/Wales)
3. Adam Jones (Ospreys/Wales)
4. Simon Shaw (London Wasps/England)
5. Paul O'Connell (Munster/Ireland) (capt)
6. Tom Croft (Leicester Tigers/England)
7. David Wallace (Munster/Ireland)
8. Jamie Heaslip (Leinster/Ireland)
16. Ross Ford (Edinburgh/Scotland)
17. Andrew Sheridan (Sale Sharks/England)
18. Alun Wyn Jones (Ospreys/Wales)
19. Martyn Williams (Cardiff Blues/Wales)
20. Harry Ellis (Leicester Tigers/England)
21. Ronan O'Gara (Munster/Ireland)
22. Shane Williams (Ospreys/Wales)