Temba Bavuma hit a century on his one-day international debut and JP Duminy took four wickets as South Africa recorded a comfortable 206-run victory over Ireland at Willowmore Park, near Johannesburg.
Bavuma was dropped by Kevin O'Brien at second slip after scoring just one before going on to make 113 off 123 deliveries, including 13 fours and a six, as the hosts reached 354 for five.
Fellow opener Quinton de Kock scored 82, while Duminy finished unbeaten on 52 and Farhaan Behardien also contributed a rapid half-century.
Facing a run-rate of more than seven an over, Ireland captain William Porterfield was trapped leg before by Kagiso Rabada off the second ball of the tourists' reply.
O'Brien added 41 and opener Paul Stirling 40 but Ireland were bowled out for 148, with Duminy producing a career-best four for 16.
Ireland next face Australia on Saturday.