Jackson, who has 13 Ireland caps, flourished under Les Kiss' tutelage during the back end of Ulster's Guinness PRO12 campaign and starts his first Test since last year's World Cup warm-up game against Wales last August.
And Ulster teammate Gilroy - who has been rewarded for his ten Guinness PRO12 tries this season with a bench spot to face an experienced South African side in their first game under new coach Allister Coetzee - has backed Jackson to shine.
"He's been fantastic. He's been a joy to play with," he said.
"It's not just off the back of this season that I say this, I played with Jacko at school, underage level, Under 19s, U20s, provincially and internationally.
"He's a joy to play with, I've always rated him so highly. I always enjoy playing with him and no doubt if he gets an opportunity I think he'll take it.
"I think this has probably been his best season on a personal level.
"Not only was he able to deliver on the pitch physically, but he stepped up as a leader. He spoke well to us guys post-game, at half-time and he talks well on the pitch and commands the plays that he wants."
Joe Schmidt has included Jared Payne at full-back for the very first time - his other 14 caps have all been won in the centre - with Guinness PRO12 winner Robbie Henshaw moving to outside centre and Luke Marshall returning to start at 12 for the first time in two years.
Gilroy is clearly confident in his fly-half's ability to excel in all departments - something he will need to come good on against a Springboks side featuring Willie le Roux, JP Peterson, Duane Vermeulen, Francois Louw, and Tendai Mtawrira.
"He delivers with ball-in-hand, his kicking game has been fantastic as well. Les knows Jack is more than capable," he added.
"He has that smart rugby brain to make the decisions to take the ball to the line or if he decides to go himself, do you know what he's a tight wee man and he's a lot quicker than he looks.
"He's hard, Jacko. Sometimes he has a laugh at himself, but he'll get down to the nitty-gritty stuff and put his body on the line."
Ireland XV to face South Africa
15. Jared Payne (Ulster)
14. Andrew Trimble (Ballymena/Ulster)
13. Robbie Henshaw (Buccaneers/Connacht)
12. Luke Marshall (Ballymena/Ulster)
11. Keith Earls (Young Munster/Munster)
10. Paddy Jackson (Dungannon/Ulster)
9. Conor Murray (Garryowen/Munster)
1. Jack McGrath (St Marys/Leinster)
2. Rory Best (Banbridge/Ulster) (captain)
3. Mike Ross (Clontarf/Leinster)
4. Iain Henderson (Ballynahinch/Ulster)
5. Devin Toner (Lansdowne/Leinster)
6. CJ Stander (Munster)
7. Jordi Murphy (Lansdowne/Leinster)
8. Jamie Heaslip (Dublin University/Leinster)
Replacements
16. Sean Cronin (St Marys/Leinster)
17. Finlay Bealham (Corintians/Connacht)
18. Tadhg Furlong (Clontarf/Leinster)
19. Ultan Dillane (Corinthians/Connacht)
20. Rhys Ruddock (St Marys/Leinster)
21. Kieran Marmion (Corinthians/Connacht)
22. Ian Madigan (Blackrock/Leinster)
23. Craig Gilroy (Dungannon/Ulster)
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