Sean Cronin has emerged as a doubt for Ireland’s third test series decider against the Wallabies in Sydney’s Allianz Stadium (kick-off 8.05pm local time/11.05am Irish time). The Leinster hooker sat out Friday’s captain’s run in the match venue due to a tightness in his quad.

If he is ruled out, it would be a desperate blow for the 31-year-old, who had been chosen for only his 10th start in 63 test matches. Having shouldered much of the blame for the 68th minute scrum in the first test which led to Australia regaining the lead for the last time, he had been omitted altogether from the second test, meaning this game represented a huge opportunity for him.

It would also mean Niall Scannell being promoted from the bench to start his second test in a row, with Rob Herring coming into the match-day squad as one of the replacements.

This third test will also see captain Peter O’Mahony and Jack McGrath both run out for their 50th tests for their country.

True to type, O’Mahony was not of a mind to allow too much self-reflection on reaching the landmark.

“It’s a hard thing to do when you’re in a series like this. It’s three of the biggest games of your life coming in three consecutive weeks - it’s hard to sit back and think about it.

“That’s not to say I’m taking it for granted, it was a dream to get my first cap and to think I’d be sitting here getting 50 is not something I could have comprehended on that day. It’s certainly a huge honour.”

With McGrath also reaching the milestone, O’Mahony said of the recalled loose-head: “His set-piece work is second to none in the world, the energy and drive he brings around the park is incredible, he’s incredibly fit for a big, big man and he’s an incredible character. That’s something that’s a big part of our group, he leads from the front.”

Peter O’Mahony will win his 50th cap this weekend. Photograph: Dan Sheridan/Inpho O’Mahony himself will be wearing the seven jersey as opposed his more unfamiliar six, and admitted he couldn’t remember the last time he did so.

“It’s been a while but at this level it doesn’t change a huge amount. CJ (Stander) would be well able to play seven as well. It changes a couple of small little roles but it’s something I was comfortable with from earlier on in the week when they asked me to slot in. Any opportunity you get to start in an Irish jersey, I’d take playing in the front row, because it’s a special honour.”

Joe Schmidt had ventured that Ireland will undoubtedly need their best performance of the series to win, and O’Mahony concurred.

“We need to build on our performance from last week and that’s the main priority for us. There were things we did a lot better last week that are going to have to step up again this week because we know how proud Australia are.

“Their record at home is incredible and a team of their calibre, their performances, we have no doubt it’s going to be one of the best they’ve put in all year. We know we have to match that and go beyond if we want to get a win tomorrow.”

Ireland comes into this game ranked second in the world on the back of winning a Grand Slam, and while that has a greater historical cache, O’Mahony said winning a series down under would rank alongside that achievement.

One of the primary concerns for the Irish squad facing into this game is that the Wallabies scored three tries off 30 per cent possession last week, and have outscored Ireland by five tries to two in the two games to date.

“I think we have to talk about it,” admitted O’Mahony. “One of their biggest assets is their ability to play rugby and to strike wide. And 30-35 per cent possession and three tries is a very impressive stat. We know how dangerous they are with the ball. As a result I think our defence needs to step up a notch.”

The try that possibly hurt the forwards the most was the Wallabies’ second try, which was a penalty try off a driving maul.

“It’s something you certainly don’t plan for, it’s something we pride ourselves on,” said O’Mahony. “You’re playing against one of the best packs in the world. Sometimes these things happen. The quality of players they have up front is second to none, you try and plan for things; sometimes they don’t work and sometimes you’re just out-done in a lineout or a maul.

“It certainly is an area where we need to improve, I think our maul defence in general and our lineout defence and attack - we were out-performed in all three areas last week.

So it’s something we really pride ourselves on and we want a better performance there.”