Injury-hit Ireland could be forced into further changes to face Australia on Saturday if Sean O'Brien and Jared Payne fail to beat "lower-limb tweaks".

Flanker O'Brien and centre Payne sat out Friday's captain's run training session, but Ireland assistant coach Andy Farrell insisted the influential duo are expected to be passed fit.

Ireland drafted flanker Peter O'Mahony back into the squad on Friday to cover O'Brien, while Munster utility back Rory Scannell would step up should Payne fail to recover.

O'Brien and Payne face fitness tests on Saturday morning ahead of the Dublin clash with the Wallabies, with Ireland already missing the injured inside-back duo of Johnny Sexton and Robbie Henshaw.

"We've had a couple of guys that's had a couple of niggles, and the captain's run is normally just a stretching-legs type session," said Farrell.

"We made a couple of decisions this morning to rest Jared Payne and Sean O'Brien and see how they are in the morning.

"We'll have a discussion in the morning about their situation.

"They both have just lower-limb tweaks, that's what normally happens during the week.

"You pick things up on the way from the game, through the week and so on.

"The main session was yesterday and we decided to give a couple of players more time.

"We've kept Peter O'Mahony here, and we've got Rory Scannell as well covering for Jared Payne.

Sean O'Brien is expected to be declared fit for Saturday's match (Getty)

"All the main work's been done already, that's why we are resting them, to give them every chance to feel 100 per cent."

Ireland must already face an experienced Wallabies line-up without the regular inside-backs combination of Sexton and Henshaw.

British and Irish Lions fly-half Sexton misses out with a hamstring problem suffered in last weekend's 21-9 defeat to New Zealand.

Powerful centre Henshaw suffered concussion in the Dublin loss to the All Blacks, leaving Garry Ringrose slotting in for his first-ever senior level start at number 12.

Munster's 22-year-old utility back Scannell will now offer cover for Ulster's Payne - the man who captains Ireland's back line defence.

Payne remains vital to Ireland's defensive system and shape, and Ireland will be desperate for the 31-year-old to come through, especially with the inexperienced Ringrose slotting into the side.

Farrell however has insisted the late doubts have not caused Ireland any real disruption.

When asked if the fitness doubts over O'Brien and Payne had affected preparations, Farrell said: "No it hasn't, because this is what we've been building for the last year.

"We've been building strength in numbers, strength in depth is getting bigger and better all the time.

"We've seen that across the last year and on the South Africa tour and we're seeing it again."

Captain Rory Best will become the fifth Irishman to win 100 Test caps on Saturday, following in the footsteps of Brian O'Driscoll, Paul O'Connell, Ronan O'Gara and John Hayes.

Assistant coach Farrell hailed Best as producing the best rugby of his career, despite hitting 34 years of age - and has taken his captaincy to new heights this autumn.

"I think he's outstanding," said Farrell of Best. "I can actually say I think he's playing better than ever, because I've seen it over the last six months.

"Captaincy at this level is a tough old gig. He took over the Six Nations last year, filling in big shoes.

"I was on tour in South Africa I was very impressed with his leadership then. And now we've come to the autumn series and it's gone up another level.