James McKinney has been promoted to the starting line-up in the out-half position, with Paddy Jackson named in the replacements.
McKinney's haul of three penalties against England saw him become Ireland's record points scorer in Junior World Championship history. He now has 46 points, two more than previous top scorer Ian Porter.
Daniel Qualter has been named alongside Michael Kearney in the second row, and Connacht's Aaron Conneely will make his tournament debut in the back row.
Iain Henderson, Dominic Gallagher and Luke Marshall have dropped to the replacements bench for Ireland's second Pool C fixture.
Andrew Conway's opportunist try against England continued his fine strike rate in this competition. A member of last year's squad also, Conway has now scored six tries in six Junior World Championship matches.
Speaking about the selection, Mike Ruddock said: "We have looked closely at South Africa and they present another massive challenge. Typically they are a big and very physical side. Combine that with a four-day turnaround and it is important that we introduce some freshness into our team.
"As such I have made five changes to the team that played England. The players coming in are all experienced age grade internationals and they will be keen to perform on the big stage."
Tuesday's game will be broadcast live on Sky Sports 3. Coverage begins at 7pm Irish time. Ireland's opponents South Africa opened the tournament with a 33-0 victory over Scotland and currently top the pool.
IRELAND UNDER-20 Team & Replacements (v South Africa Under-20s, IRB Junior World Championship Pool C, Stadio Comunale di Monigo, Treviso, Tuesday, June 14, kick-off 8.10pm local time/7.10pm Irish time):
15 - Tiernan O'Halloran (Galwegians/Connacht)
14 - Andrew Conway (Blackrock/Leinster)
13 - Brendan Macken (Blackrock/Leinster)
12 - JJ Hanrahan (UL Bohemians/Munster)
11 - Craig Gilroy (Dungannon/Ulster)
10 - James McKinney (Queen's University/Ulster)
9 - Kieran Marmion (UWIC/Exile)
1 - James Tracy (UCD/Leinster)
2 - Niall Annett (Belfast Harlequins/Ulster) (capt)
3 - Tadhg Furlong (Clontarf/Leinster)
4 - Michael Kearney (Clontarf/Leinster)
5 - Daniel Qualter (Buccaneers/Connacht)
6 - Jordi Murphy (Lansdowne/Leinster)
7 - Aaron Conneely (Galway Corinthians/Connacht)
8 - Eoin McKeon (Galwegians/Connacht)
16 - David Doyle (UCD/Leinster)
17 - Conor Carey (Ballymena/Ulster)
18 - Iain Henderson (Queen's University/Ulster)
19 - Dominic Gallagher (Dublin University/Ulster)
20 - Peter du Toit (UCD/Leinster)
21 - Paddy Jackson (Dungannon/Ulster)
22 - Luke Marshall (Ballymena/Ulster)
IRELAND UNDER-20 POOL MATCHES (all kick-off times are local time) -
Friday, June 10: England 33 Ireland 25, Stadio Comunale di Monigo, Treviso
Tuesday, June 14, 8.10pm: Ireland v South Africa, Stadio Comunale di Monigo, Treviso (live Sky Sports 3)
Saturday, June 18, 8.10pm: Ireland v Scotland, Stadio Mario Battaglini, Rovigo