Victory over the Wallabies would all but clear Irish passage into the knockout stages of the World Cup with Italy the final major hurdle to negotiate.
The odds are heavily stacked against them, however, with Australia positioned as the tournament's second favourites and Ireland's plans disrupted today by a tournament-ending injury to Jerry Flannery.
Flannery had been pushing starting hooker Rory Best hard after some impressive substitute appearances but will now be flying home to be replaced by Damien Varley, with Sean Cronin moving up to the bench.
Ireland laboured to a 22-10 win against the United States in their Pool C opener last Sunday, with Australia crushing Italy 32-6.
The form book points to a routine assignment for Robbie Deans' side, but D'Arcy refuses to consider anything other than a stunning upset.
"I think we're capable of beating Australia. I wouldn't be here if I didn't think so," said the Leinster centre.
"We can do as much video and prepare as best as we can, but if you don't believe you're already on the back foot.
"Belief is a massive, massive thing in sport. You must have belief in yourself and the belief in the guys around you.
"I believe that across the board we can match and beat them.
"Australia are the form team coming into the World Cup. We wouldn't underestimate them at all.
"Quade Cooper is one of the outstanding backs in the world this year. Pat McCabe has given them that directness they've been looking for.
"Adam Ashley-Cooper has been phenomenal. They are a very good team and it would be foolish to think otherwise.
"But in the same breath there's no point me saying I don't think we can beat them. We can't put them on a pedestal."
Ireland's failure to launch their campaign with a bonus-point victory over the USA was disappointing, but D'Arcy accepts the backs must shoulder much of the blame.
"We weren't happy with the way we played. We obviously let the forwards down because they had a pretty good game," he said.
"It was a good wake up call for us. We should have kicked on more, but we lacked that bit of accuracy and clinical touch."
Ankle surgery prevented D'Arcy from participating in Ireland's build up to the World Cup until the latter half of last month.
Appearances in the warm-up fixtures against France and England and last weekend's run-out in New Plymouth have given him the belief that he is close to regaining form.
"I'm feeling really good. Last weekend was the best I've felt in a game," he said.
"I'm happy with where I'm going. I'm not where I want to be form wise, but hopefully this week will be another step up the ladder.
"I've had three games under the belt now. We worked out a plan of when I'd be peaking and hopefully it will be on Saturday."
Commenting on the injury to Flannery, whose long-term future is now uncertain, Ireland team manager Paul McNaughton said: "Obviously it is particularly disappointing for Jerry to have suffered this injury at this point in the tournament.
"With the fact that he is out for four to six weeks, we needed to make the decision to fly Damien Varley down as soon as possible.
"Sean Cronin will step into the replacements seamlessly this weekend which shows the importance of having experience and depth in the squad."
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