Saturday's Rugby World Cup game between champions South Africa and Ireland will be a unique one for Springbok lock Jean Kleyn.

The 30-year-old could be in line for his sixth test appearance for his country of birth after he was picked on the bench for the potential Pool B decider.

Four years ago during the last World Cup in Japan, Kleyn won the last of his five caps for Ireland.

This World Cup is the first edition of the competition since laws changed to allow a player to feature for a second country they qualify to represent, following a stand-down period from international rugby.

"The two are quite detached to me. I was a different person back then," Kleyn told reporters after Sunday's 76-0 win over Romania.

"I was much younger, I paid a lot more attention to what people said compared to now, especially in the media.

"There was massive pride for me to represent Ireland back then and now I have a massive opportunity to be able to represent my country of birth," the Munster second-row added.

The likes of ex-All Blacks Lima Sopoaga and Charles Piutau and former Australia fly-half Christian Leali'ifano have also benefited from the evolution of the rule to now turn out for Pacific Island countries.

Some have criticised the move from World Rugby, including the body's former vice-chairman and ex-Argentina scrumhalf Agustin Pichot.

"It's a lot of outside noise to me, I try to focus on what happens inside the team and my family," Kleyn said.

"I try to focus on the internal conversations I'm having.

"I know the reasons why I do things and I know those reasons will change," he added.

'DRIVE PRECISION'

Kleyn moved to the Irish province in 2016 to work under current South Africa director of rugby Rassie Erasmus and their head coach Jacques Nienaber.

Erasmus led South Africa to the 2019 World Cup title but Nienaber is now the man in charge, with the Springboks having claimed two wins in as many games as they begin their tilt at a record fourth Webb Ellis Cup victory.

This weekend in Paris, the winner of Pool B is likely to be decided, with Kleyn set to come up against his former test match captain Johnny Sexton and a host of his Munster teammates, such as flanker Peter O'Mahony.

Ireland are also unbeaten so far and top the group, with a tough quarterfinal most probably against in-form hosts France or three-time winners New Zealand on the horizon for both sides, should they make the knock-out stages, as they are expected to do.

"They're an incredibly clinical team, that's the thing with the Irish, they tend to nail the details," Kleyn said.

"Guys like Johnny and Peter are quite serious about driving the detail and not making small mistakes.

"They drive precision and execution. We'll have to pitch up on the day," he added.