Warming up for a five-match series against Australia, South Africa racked up 354-5 and bowled out the Irish for 148 in a convincing start to their home season.
Normally a middle-order batsman, Bavuma was selected as an opener for his first ODI and responded with 113, sharing a 159-run opening stand with Quinton de Kock (82).
South Africa pounced on Ireland's decision to put the home team in to bat. JP Duminy added 52 not out and Farhaan Behardien made 50 from 22 balls.
Duminy took 4-16 with his offspinners and Ireland lasted just 30.5 overs. Ireland plays Australia on Tuesday.