Ruddock called on his talented squad to shock the home side and they duly did, led by man-of-the-match, Munster's JJ Hanrahan who kicked 13 crucial points and skilfully pushed Ireland forward in open play.
Kerry native Hanrahan converted Jordan Coghlan's 27th-minute opening try to give the visitors a 13-3 half-time lead which was founded on a solid set piece game and muscular defending.
Ireland looked much sharper up to that point, but predictably the South Africans clawed their way back into contention.
A close range try from lock Paul Willemse gave them momentum, while out-half Tony Jantjies was assured from the kicking tee. He converted Willemse's effort and also landed his second and third penalties to make it 16-16 just past the hour mark, with Hanrahan sandwiching in a smartly-taken drop goal.
Ireland skipper, Munster's Niall Scannell was part of a hugely determined Irish pack and they took everything South Africa could throw at them in a tense final 20 minutes.
Ireland's ability to charge down the home half-backs was a feature throughout this Pool B opener, and one such incident led to a timely second try for Ruddock's side.
Replacement hooker Munster's James Rael blocked an attempted clearance kick and retrieved possession before being driven up close to the try-line. Iain Henderson picked off the base and fend off two initial defenders and stretched over past a third for an inspirational try.
Hanrahan held his nerve brilliantly to add the conversion from the left and Ireland the benefit of a 23-16 cushion heading into the closing stages.
The Junior Springboks had the better of both possession and territory as they tried desepartely to hit back however a fourth penalty from Jantjies was their only reward.
Ireland stood firm to record the stand-out result of the opening round. They take on England in their next Pool B match on Friday, kick-off 5.45pm. This match will be televised on Sky Sports 2 and TG4.
IRELAND UNDER-20 Team & Replacements
15. Peter Nelson (Dungannon/Ulster)
14. Barry Daly (UCD/Leinster)
13. Foster Horan (Lansdowne/Leinster)
12. Chris Farrell (Dungannon/Ulster)
11. Sam Coghlan Murray (UCD/Leinster)
10. JJ Hanrahan (UL Bohemians/Munster) * Vice Captain
9. Kieran Marmion (Corinthians/Connacht)*
1. Des Merrey (Clontarf/Leinster)
2. Niall Scannell (Dolphin/Munster) Captain
3. Tadhg Furlong (Clontarf/Leinster) *
4. Tadhg Beirne (Lansdowne/Leinster)
5. Iain Henderson (Queen's University/Ulster) *
6. Jordon Coghlan (UCD/Leinster)
7. Jack Conan (Old Belvedere/Leinster)
8. Conor Gilsenan (UCD/Mullingar/Leinster)
16. James Rael (Garryowen/Munster)
17. Peter Reilly (Lansdowne/Leinster)
18. Jake Cawley (Old Belvedere/Leinster)
19. Alan O'Connor (UCD/Leinster)
20. Aaron Conneely (Corinthians/Connacht) *
21. Luke McGrath (UCD/Leinster)
22. Jake Carty (Buccaneers/Connacht)
23. Mikey Sherlock (Clontarf/Leinster)