South Africa needed a gutsy half-century from all-rounder Andrew Hall to reach a face-saving 192 after being reduced to a pathetic 91-8 by seamers Trent Johnston and Dave Langford-Smith.
Australia-born Johnston finished with 4-40 and Langford-Smith with 3-30.
South Africa had the bowlers to defend their modest total as they dismissed Ireland for 157, with Charl Langeveldt grabbing four wickets and Hall three to help their team win with more than five overs to spare.
Ireland gave a scare or two to South Africa as they were strongly placed at 139-4 at one stage before losing their last six wickets for 18 runs.
Middle-order batsman Andre Botha top-scored for Ireland with 40, while Kevin O'Brien contributed 33. The pair put on 54 for the fifth wicket.
Opener William Porterfield was the other notable scorer with 37.
"I am proud of the guys. They put up a brilliant effort and hopefully they carry on like this in the coming games," said Irish skipper Johnston.
"We did not have the firepower to get the last two wickets and this gave us some good lessons to bowl the death overs better. Hall has opened the batting for South Africa and he is a perfect number nine to be in a side."
Ireland may have lost the match but gave a morale-boosting performance against a stronger outfit ahead of the mega one-day event, starting at Jamaica on March 13 with a match between Pakistan and the West Indies.
"We got ourselves in a bit of trouble, but I knew we could do it with a big partnership. I'd have preferred it to come from top of the order," said South African coach Mickey Arthur.
"We got to be slow in the tournament because we did not have a proper bat-out in the past two weeks. But I hope we peak at the right time."
Little went right for South Africa after winning the toss in their opening practice game as they lost eight wickets for 91 runs against the disciplined pace duo of Langford-Smith and Johnston.
Langford-Smith rocked the top order with three early wickets, while Johnston kept pressure on the middle order.
Hall spared South Africa the batting blushes with a responsible 67 not out under pressure, hitting one six and six fours in his 98-ball knock.
He received valuable support from Robin Peterson, who contributed 29 in an 85-run stand for the ninth wicket.
Skipper Graeme Smith, AB de Villiers and Ashwell Prince failed to reach double-figures, while Herschelle Gibbs (21) could not capitalise on the start.
South Africa are in Group A with defending champions Australia, Scotland and the Netherlands, while Ireland are in Group D with the West Indies, Pakistan and Zimbabwe. The top two sides from each group will advance to the next round.
South Africa 192 all out in 50 overs (Andre Hall 67, Peterson 29, Gibbs 21; D Johnston 4 for 40, Langford-Smith 3 for 30, Andre Botha 2 for 24) beat Ireland 157 all out 44.2 overs (Andre Botha 40, Porterfield 37, K J O'Brien 33; Charles Langeveldt 4 for 31, Andre Hall 3 for 26, Andre Nel 2 for 38) by 35 runs.