Captain Paul O'Connell was relieved that Ireland edged past Italy to reach the Rugby World Cup quarter-finals, but warned they must improve to beat France and finish top of Pool D.

Ireland battled to a 16-9 victory at London's Olympic Park on Sunday but were well below their best against the Azzuri, with Keith Earls scoring their solitary try to become his country's top World Cup try scorer.

Jonathan Sexton also added 11 points with the boot, but arguably the most important contribution came from Peter O'Mahony, whose try-saving tackle on Joshua Furno prevented Italy from taking the lead early in the second half.

Ireland need to beat France to avoid a likely last-eight meeting with world champions and favourites New Zealand and O'Connell is aware that Les Bleus will punish another error-strewn showing.

"It's a massive game [against France], they're playing good rugby at the moment and we have to improve our performance," he said.

"We're into the quarter-finals and that's a bonus. We always saw it as a knockout game.

"It was a disappointing performance. We were inaccurate and put too many balls down. When we do that we can't play our game.

"It interrupted our momentum and gave them hope. When you hand them ball on a plate they'll do damage, only a try saving tackle from Peter O'Mahony kept us in the game."