Ireland, who lost to England by 48 runs in their first Super Eights match, now meet New Zealand in their next match here on April 9.
Jacques Kallis hit an unbeaten 66 and added 80 runs for an unbroken fourth wicket stand with Ashwell Prince (47 not out) to guide the Proteas to their second win in as many Super Eight matches.
South Africa, who restricted Ireland to 158-8, were set a revised target of 160 after two rain interruptions of nearly three hours and a 15-minute delayed start, reduced the match to 35 overs per side.
Graeme Smith's side, who achieved a narrow nerve-tingling one-wicket win over Sri Lanka here last week, now meet Bangladesh on Saturday before their crucial matches against the West Indies, New Zealand and England.
Ireland, who gave another good account of themselves, could have made a match out of it had they not dropped Kallis twice.
Kallis, when on 40, was dropped by paceman Boyd Rankin off his own bowling, while Trent Johnson grassed a paddle sweep off Kyle