Bavuma, dropped on one, scored 113 as South Africa reached 354 for five with Farhaan Behardien hitting 50 off 22 deliveries and Quinton de Kock (82) and Duminy (52 not out) also contributing runs.
Ireland lost captain William Porterfield to the second ball of their innings, trapped lbw by Kagiso Rabada, and were always up against it chasing a run rate of over seven an over.
Kevin O’Brien (41) and Paul Stirling (40) provided some resistance as Ireland reached 148 before being bowled out with 19 overs to spare.
Duminy returned career-best figures of 4-16. Ireland, given a rare chance to play against top tier opposition, next play Australia at the same venue, on the outskirts of Johannesburg on Tuesday. Australia and South Africa will then embark on a five-match ODI series, starting on Friday.
