Steyn made 84* off 104 balls and Goodall 93* off 99 as South Africa reached their target of 214 in 38.4 overs. Laura Wolvaardt (27 off 29) was the only Proteas batter to lose her wicket.
Earlier, Ireland posted 213-8 in their 50 overs, thanks to captain Gaby Lewis (59), Sophie MacMahon (42) and Georgina Dempsey (45*).
Shabnim Ismail was South Africa’s best bowler with figures of 3-31 in 10 overs.
The Proteas will take an unassailable 2-0 lead into the final ODI in Dublin on Friday.