The hosts posted 143-7 in their 20 overs thanks to an opening partnership of 98 between Leah Paul (47) and captain Gaby Lewis (52).
Proteas seamer Tumi Sekhukhune took 3-32 in her four overs.
In reply, the visitors made 133-7 with Anneke Bosch (29), Laura Wolvaardt (20), captain Suné Luus (23) and Chloe Tryon (26) all making useful contributions without kicking on.
Arlene Kelly (2-25 in four overs) was Ireland’s best bowler.
The second T20I of the three-match series will take place at the same venue on Monday.
