The home side led 12-3 at the point where Jamie Heaslip was sin-binned early in the second half, and were taken apart in his absence. Heaslip was captaining the team for the first time, and South Africa prevailed 16-12.
Pienaar crossed for the only try of the match in the 45th minute and Pat Lambie kicked 11 points as South Africa, who started as strong favourites, took control.
Pienaar, who plays club rugby for Ulster, said: "I think we had a terrible first half. Our kicking game wasn't that great. In the second half our forwards made a huge step up.
"We got a couple of good drives going, kept the ball and forced them to make errors. I'm very proud of the guys, especially in the second half."
Asked if there was any sense of concern that South Africa had managed just one try, Pienaar said on BBC2: "Not really. It's a hard place to come and play rugby. It's a good Irish team.
"To get just one try is the least of our concerns."
Gilbert has released a new line of rugby cleats. The Gilbert Virtuo 8S is part of the exciting new product. Check it out.
The entire All Blacks apparel line has been updated for 2013/14. Check out the New Zealand All Blacks polo.
The Nike Tiempo is a solid rugby cleat and one of few styles still made from full-grain natural leather.
The Gilbert Blitz 8S rugby cleat is a great cleat at a great price of $69.99. Get a new pair of cleats today.
A cool looking all black rugby cleat with the high performance adidas is known for. Get in the Gear!
Wear the crest of the British and Irish Lions on your t-shirt. A great look for the summer.
The All Blacks Performance t-shirt is black with hints of blue from the training jersey. Very Cool.
The New Zealand All Blacks training jersey for 2013/14. Get in the Gear!
The USA Rugby Pro Alternate rugby jersey is perfect for any fan of the Eagles. Get yours to wear during the summer Test matches.
The NEW All Blacks 2013/14 jersey has arrived at World Rugby Shop. Dare to wear the colors of the All Blacks.