The idea that winning the World Cup is not a priority for international rugby's most in-form team may not quite make sense.

But it is hard to argue against a culture which prides improving performance above all else - including results - when in the last year it has achieved the best win percentage of any tier-one team on the planet.

Such is the case for Ireland, who head into the Six Nations this weekend as the team to beat, following an unbeaten autumn campaign during which they defeated world champions New Zealand.

Following that loss, All Blacks coach Steve Hansen anointed Ireland as World Cup favourites, but former Irish captain Paul O'Connell believes lifting the Webb Ellis Cup for the first time will not define their ambitions.

The veteran of four World Cups says the focus will be firmly on performance, a culture developed from age-grade sides to Test level that can make Ireland the best in the world.

"New Zealand, when you look at the strength and depth they have, the record they have, I'd have them as number one," he told BBC Sport. "But Ireland, with smaller numbers, are pushing them very close. I do think they have the capacity to be the best.

"This is the first time they go to a (World Cup) tournament having beaten every leading team fairly recently.

"But it's funny, there'll be no goal-setting done where they'll see winning the World Cup as the definition of success. The definition of success for them will be improvement and if they keep improving the way they are, they could, and should, win it."

Vunipola return threatens Six Nations hopes

First up, though, Ireland must defend their Six Nations title, a championship O'Connell, who played in 13 campaigns, believes is "so important". Ireland sealed the Grand Slam last year, only a third in their history, against England at Twickenham, and they open this campaign against the same opponents in Dublin.

While visiting head coach Eddie Jones regards the hosts, who have won 18 of their last 19 Tests including 12 in a row at the Aviva Stadium, as the best team in the world, he has also promised a brutal contest.

Ireland could secure three straight wins against England for the first time in eight years, but BBC TV rugby expert O'Connell believes they will face "a very different team" on Saturday.

England will be motivated by the pain of their worst Six Nations finish of fifth in 2018, and bolstered by the return of 130kg number eight Billy Vunipola. The Saracens forward could make just a fifth Test appearance in nearly two years following a series of injuries.

Billy Vunipola's last England cap came against South Africa in June 2018 but he has now recovered from a broken arm

"In my final year at Munster I played against Billy for Saracens and he was without a doubt one of the best players I ever played against," said Ireland's third-most capped player, who won eight of his 13 meetings with England.

"A lot of guys his size and shape tend not to be fit enough to have a relentless effect on the game, but I thought he was incredible. A brilliant ball carrier, very hard to tackle, great feet, can pass. He's going to make a really big difference."

The former British and Irish Lions captain also hailed the influence of "world class" England fly-half Owen Farrell, a team-mate on the 2013 Lions tour to Australia "who just makes the guys around him better".

But, despite claims from the England camp the pressure will be on Ireland to win, the tournament favourites will only be focused on one thing.

"I think Eddie Jones probably thinks he's putting Ireland under pressure but pressure doesn't overly bother this team," added O'Connell. "It's not all about winning, it's about that performance."

School classroom to super coach for Schmidt

The "long-term investment" in continual improvement has allowed the current coaching set-up to "reap the rewards", according to O'Connell.

In six years, head coach Joe Schmidt has led Ireland from eighth to second in the world rankings, while Ireland have won 74% of their matches (46 of 62) during his tenure. Their win rate in the professional era before his appointment was just 54%.

Schmidt is set to retire after the World Cup, to be replaced by assistant coach Andy Farrell, following three Six Nations titles, a Grand Slam, two wins over New Zealand, a first victory against South Africa and first series triumph in Australia.

Current captain Rory Best has lauded Schmidt's development of fellow coaches, but to O'Connell, his biggest strength is a skill first honed teaching high school English in his native New Zealand.

"He's a brilliant teacher," said O'Connell, who captained Ireland to their Six Nations victories under Schmidt in 2014 and 2015. "Coaches can develop those habits and behaviours in their players quicker than others, and the ability to develop quicker is down to your ability to teach quicker."

The analytical approach of the two-time Heineken Cup winner has resulted in Ireland's execution of basics becoming a ruthless asset. They conceded the fewest penalties and fewest turnovers of any tier-one team in 2018. If accuracy is a buzz word of Test rugby, Ireland rarely miss.

"When they have to kick, they kick well, their set-piece is excellent," said O'Connell.

"Generally, the line-out has been top-class as well. Their catch-pass, in the backs and the forwards, is getting to the stage where it rivals New Zealand.

"Collectively their basics are as good as anyone's at the moment."

Best system in the world means match-sharp players

In Ireland, players are centrally contracted to the union, not clubs, meaning time on the pitch can be carefully managed to keep international players fresh for the Test arena.

Farrell and Irish number 10 Johnny Sexton are likely opponents this weekend. Sexton has played 552 minutes of club rugby this season, while Farrell, at 1,015 minutes, has played nearly double.

While O'Connell says Ireland have to be "smart" because of a significantly smaller playing pool than some of their rivals, he believes they have, along with New Zealand, the best system in the world for managing players.

"By the time the Six Nations comes round, the players are fresh but they're also match-sharp," he said. "It means players are good for their provinces but also good for Ireland."

Jacob Stockdale scored the only try when Ireland beat the All Blacks 16-9 at the Aviva Stadium in November

Ireland have also developed formidable strength in depth, with the IRFU able to ensure promising players are blooded in the Pro 14 as part of their development.

The rise of players like Munster back-row Tadgh Bernie, club-mate and Heineken Cup top points-scorer Joey Carbery and Connacht scrum-half Kieran Marmion, means that although Sexton remains hugely important, Ireland's reliance on star players is reducing.

But if a full-strength Ireland played New Zealand in a World Cup final tomorrow, would O'Connell back them to win? "I would, but then, I'm biased."