Ireland were expected to put up a fight against the Proteas and they did meet the expectations for some time with the ball.
Duminy came in and arrested the slide and took South Africa to safety. Ireland never really recovered after the early strikes by Morne Morkel.
To their credit the South Africans never gave them any breathing space and kept it very tight throughout with their bowling as the minnows were all out for only 141 in 33.2 overs.
Ireland will be pleased with their bowling effort though there still seems to be slight concerns over the performance of the top order. Ireland seem to have run out of steam and will do well to win the game against Netherlands.
South Africa are through to the quarters and seem to be peaking at the right time.
Ireland sunk into real trouble as Robin Peterson struck twice in his fifth over to remove Gary Wilson and big-hitting Kevin O'Brien.
There was no magic from Kevin O'Brien this time as he went for a big shot of a Robin Peterson ball and ended up lofting it straight to long off where Amla took a dolly.
Kallis claimed Niall O'Brien after Morne Morkel struck in his consecutive overs to dismiss Ireland openers.
Paul Stirling gave a thick edge to a short pitched ball of Morkel and Kallis took a simple catch in the slip.
Earlier, JP Duminy missed out on a century but rescued a shaky South Africa batting effort to drag the Proteas to 272-7 against the minnows.
Duminy topscored with 99 from 103 balls, sharing an 87-run stand with World Cup rookie Colin Ingram (46) and a half-century partnership with Johan Botha after a fighting Irish performance in the field had South Africa struggling at 117-5.
George Dockrell took a fine diving catch on the boundary to dismiss opener Hashim Amla for 18 and Graeme Smith (7) and Jacques Kallis (19) were both run out.
Morne van Wyk hit a quickfire 42, but also fell early in South Africa's carefree start, before the composed Duminy dragged the Proteas to a respectable total.
Duminy hit six fours and a six, but played a measured knock initially after South Africa's top order had tried to be too attacking against the Irish and paid for it.
Needing to win its remaining Group B games against the South Africans and Netherlands to keep alive hopes of a place in the quarterfinals, Ireland had recovered from two missed catches and had South Africa's top order reeling.
But Duminy delivered a 13th career ODI half-century and eventually fell one short of a maiden World Cup century when he skied a shot off John Mooney (1-36) and was caught three balls from the end.
Ingram supported him at better than a-run-a-ball after being called into the team in place of injured batsman AB de Villiers.
Botha finished 21 not out, while leftarm spinner Dockrell ended with 1-37 off 10 overs and was one of five Irish bowlers to take a wicket.
