O'Sullivan said: "At the start of the summer people wrote off South Africa because of their World Cup performance and the presence of a new coaching set-up. There was a lot of uncertainty surrounding them.
"But they rallied fantastically, beat us twice, Wales once and went on to win the Tri-Nations. They are currently the best international side around.
"They have very big, athletic and physical players and it's very hard to contain them. They have pace out wide and can be a handful if they get quick ball on the gain line. They are pretty much the complete package.
"But we've always relished these contests because we enjoy testing ourselves against the best. We respect them but are not afraid of them. We know that if we execute our gameplan properly, it will be a very tight game."
Springbok coach Jake White has made three changes to his 22-man squad, and one to his starting XV.
A fully-fit AJ Venter returns at flank in place of Juan Smit, with Danie Rossouw coming on to the bench for Tim Dlulane.
Gaffie du Toit or Jaque Fourie will take the place of Brent Russell.
Fly-half Jaco van der Westhuyzen, who injured his finger in the Welsh clash, has been named but will undergo a fitness test.
Ireland: Girvan Dempsey, Geordan Murphy, Brian O'Driscoll, Shane Horgan, Dennis Hickie, Ronan O'Gara, Peter Stringer, Reggie Corrigan, Shaun Byrne, John Hayes, Malcolm O'Kelly, Paul O'Connell, Simon Easterby, Johnny O'Connor, Anthony Foley.
Replacements: Frank Sheahan, Marcus Horan, Donncha O'Callaghan, Eric Miller, Guy Easterby, David Humphreys, Kevin Maggs.
South Africa: Percy Montgomery, Breyton Paulse, Marius Joubert, De Wet Barry, Ashwin Willemse, Jaco van der Westhuyzen, Fourie Du Preez; Os Du Randt, John Smit, Eddie Andrews; Bakkies Botha, Schalk Burger, Victor Matfield, A J Venter, Joe van Niekerk,
Replacements: Hanyani Shimange, CJ van der Linde, Gerrie Britz, 19 Danie Rossouw, Michael Claassens, Jean de Villiers, Gaffie du Toit or Jaque Fourie.