The Wallabies are expecting their defence to be given its biggest Rugby World Cup work-out yet against an Ireland side planning an attacking blitz in Saturday's crucial pool A clash.

Ireland coach Eddie O'Sullivan made his intentions clear, naming hard-running Ronan O'Gara at five-eighth ahead of the more traditional David Humphreys for the match at Melbourne's Telstra Dome.

Australia has had little work to do in defence in its last two World Cup matches, chalking up 232 points against minnows Romania and Namibia.

A disappointingly conservative Argentina also did little to test the Wallabies in the opening match, but coach Eddie Jones said Ireland, now free of the pressure to make the quarter-finals, would be a different story.

He said the selection of O'Gara at No.10 signalled Ireland's plan to play with width, rather than hit it up and use Humphrey's boot to gain field position.

"It probably indicates that they're going to shift the ball around a little bit more than they have in previous games," Jones said.

"O'Gara's a very good player who tends to take the ball a bit more to the line than Humphreys so we're anticipating they may play a bit more expansive game."

Kevin Maggs recovered from an ankle injury to join game-breaker Brian O'Driscoll in the centres, which Jones said was a formidable pairing.

"With Maggs and O'Driscoll, they've got a nice set of skills to complement one another.

"Maggs is a very good one-on-one tackler and a good ball carrier while O'Driscoll tends to be more elusive so it's a good combination to have."

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