"The amount of detail they go into is incredible," said the coach who quit the Springboks after the 2023 World Cup final triumph over New Zealand and joined Irish provincial giants Leinster.
"It is microscopic. I did not do more than a tenth of the detail in South Africa that they do," he admitted in a documentary series on the latest of four Springbok World Cup triumphs.
The countries clash on Saturday in Pretoria and seven days later in Durban in a highly anticipated two-Test series to be watched by 50,000-plus sell-out crowds.
Every ticket for the international between the top ranked South Africans and second-placed Irish was snapped up within 30 minutes for the Pretoria Test.
South African supporters are hoping for a series win that will end an impressive Irish record against the world champion Springboks.
Ireland have won their last three Tests against South Africa, including a pool match at the 2023 World Cup in France, and triumphed in eight of the last 12.
Current South Africa head coach Rassie Erasmus highlighted the advantage Ireland enjoy by having a team packed with stars from 2024 Champions Cup runners-up Leinster.
"These players are performing together in the Champions Cup and United Rugby Championship," said Erasmus, who coached Irish province Munster before returning to South Africa in 2018.
'Huge advantage'
"This is a huge advantage because you do not have to spend time developing teamwork. Of the 35 players who have come here for the Test series, over half are from Leinster."
Felix Jones worked with Nienaber and then director of rugby Erasmus at the 2023 World Cup when South Africa overtook arch foes New Zealand as the most successful team in the global showpiece.
Now part of the England set-up, Jones says the Irish players "are incredibly skilful, probably up there with New Zealand, if not surpassing them.
"There is depth in the squad with world-class players in many positions, and they are not reliant on just a couple of guys."
However, despite a 2023 Six Nations Grand Slam and winning the competition this year, Ireland are constant World Cup underachievers, never progressing beyond the quarter-finals.
After defeating South Africa 13-8 at the pool stage last October, they believed they would face the defending champions again in the title decider.
Bur Ireland bowed out at the last-eight stage, losing by four points to New Zealand in a Paris thriller.
South Africa have included 12 World Cup final starters in the team for the Pretoria Test, and up to 10 of the Ireland side that fell to the All Blacks could face them.
Paris Olympics rugby sevens duty rules out full-back Hugo Keenan, winger Mack Hansen and scrum-half Jamison Gibson-Park are injured, and fly-half legend Johnny Sexton has retired.
Outside backs Jimmy O'Brien and Calvin Nash and half-backs Jack Crowley and Conor Casey are possible replacements in a side coach Andy Farrell will announce on Thursday.
There has to be at least one change among the forwards with lock Iain Henderson injured and Joe McCarthy is expected to continue partnering Tadhg Beirne in the second row.
