O'Brien: Key player

Ireland and Wales meet in the first of two all-RBS Six Nations World Cup quarter-finals with the tussle at seven sure to prove highly influential on the outcome.

It is a contrast of styles between two of the game's outstanding flankers.

O'Brien, a converted blindside, is a wrecking ball who always breaks the first tackle while Warburton is a fetcher who dominates the breakdown.

Both players have emerged as stars of the World Cup and O'Callaghan insists they are at the very top of their game.

"It's a huge battle," said the Lions and Munster second row.

"If you were putting together a world XV the two of them would be knocking on the door following their performances at the World Cup.

"Sean's been brilliant for us. He's brought a new element altogether to our game.

"Warburton is a standout player and we have to do a lot of analysis on him. He's at world-class level.

"You hear about guys like Richie McCaw and David Pocock, but this guy can do serious damage at the breakdown and in the tackle.

"We'll have to find ways to negate him at the breakdown."

Several parallels can be drawn between Ireland and Wales, among them their superb form at the World Cup and the fantastic spirit that exists in both squads.

O'Callaghan has revealed the superbly-conditioned Irish forged a strong bond during their gruelling warm-up camp at Carlton House and the players will now run through brick walls for each other.

"There's a club feel to this squad. We did pre-season training together," he said.

"You can do all the team bonding you want but when you're at Carlton House on a Friday afternoon and all you want to do is be sick, but one of the lads is encouraging you to get through, that's where you make bonds.

"We're seeing the results of that now. The fellas are willing to do anything for each other."