Willemse cupped his hand to his ear taunting the Irish supporters before Feinberg-Mngomezulu missed the conversion.
The Springboks had been reduced to 14 men in their previous two tests and were fortunate Feinberg-Mngomezulu was not sent at least to the sin bin for making no effort to tackle Tommy O’Brien and instead putting his shoulder into the wing’s head.
However, referee Matthew Carley ultimately just awarded a penalty to the hosts.
Carley, though, had no hesitation in wielding a yellow card at the experienced Ryan after he caught Malcolm Marx in the head at a ruck.
It was doubly costly as Tadhg Beirne’s try was ruled out as a result.
Carley, keeping a calm head in a febrile atmosphere, gave the Irish some relief when he ruled out a Springbok try owing to a forward pass.
That was about the only good news for captain Caelan Doris and his men, with Carley calling him over to tell him Ryan’s card had been upgraded to a 20-minute red.
Things looked dark indeed for the Irish when Reinach went over as Feinberg-Mngomezulu converted for 12-0 and Prendergast was sin-binned for repeatedly being offside.
Some sides would have wilted being down to 13 men but the Irish reacted positively, triggering huge roars around Lansdowne Rd as Dan Sheehan went over.
Crowley converted for 12-7 but minutes later he too was joining fellow first five-eighths Prendergast in the sin-bin, leaving them rudderless with no specialist in that role.
Prendergast’s brother Cian replaced Ryan, just as Crowley was trotting off, after the 20-minute red card was up, but the Irish astonishingly were down to 12 men by halftime.
Carley lost patience and sent Porter to join the two first five-eighths and then awarded a penalty try, which gave the Springboks a 19-7 halftime lead.
The Irish could not run off fast enough in case Carley decided to hand out another yellow.
Prendergast converted a penalty on his return to the fray for 19-10 but the ‘Boks hit back immediately against the 13 men.
The impressive Feinberg-Mngomezulu sashayed past Jamison Gibson-Park to go over, although he missed the conversion at 24-10.
Crowley and Porter returned to make it a 15 v 15 match, the latter coming back on just as his Leinster teammate and Springbok lock RG Snyman was sent on for his 50th cap.
Prendergast stroked over a penalty with 25 minutes remaining for 24-13 to give the hosts a glimmer of hope.
Carley’s well-used yellow card was flashed once again as McCarthy trudged to the sidelines but the sterling Irish defence turned the ball over and Gibson-Park cleared, sparking the biggest roar of the night from the home fans.