Former Ireland head coach Eddie O'Sullivan believes a good Six Nations can "copper-fasten" Ireland's good mood ahead of this year's World Cup.

Andy Farrell's side start their Six Nations campaign in Wales on Saturday.

"If the Six Nations goes badly for you then that hangover doesn't go away," said O'Sullivan.

Speaking to BBC Radio Ulster's Sportsound Extra Time, O'Sullivan, who was Ireland's head coach from 2001-2008, says the Six Nations is always "a very important tournament for Irish rugby".

"As an Irish coach, you are under no illusions that the IRFU want your best foot forward in the Six Nations," he said.

"Ireland are in rude health at the moment. We had a fantastic summer tour, where we beat New Zealand, and had a really good autumn where we were undefeated.

"Now you want to double down on that and copper-fasten the mood in the camp.

"Having a good Six Nations basically puts you in a better state of mind going to the World Cup. If you have a bad Six Nations then you have a bad six months dealing with the fallout of that,.

"You don't have much choice, you have to put your foot to the floor and get the best Six Nations you can.

"I can't speak for all Unions, but the IRFU is very clear in that it wants a good Six Nations every year, they don't care if it is a World Cup year."

Ireland 'the better team'

O'Sullivan believes Ireland cannot underestimate Wales despite the hosts being on a poor run of form, which included a surprise defeat by Georgia in November.

Ireland's last Six Nations win over Wales in Cardiff came in 2013.

He adds the return of Warren Gatland, who has come in during a "crisis situation", will also gives Wales a boost heading into the tournament.

"Cardiff has always been a hard place to go and win," he said.

"The Welsh don't travel to Dublin well, we've probably always got the better of them on an average, but when you go to Cardiff and play in front of a Welsh crowd, it's always a very tricky fixture.

"This is one we have to be really careful about. On a balance, we are a better team and we should win but games don't always go that way.

"In the Six Nations it is so important to start well. If you lose the first game then you are on the defensive straight away."

Farrell has often selected a settled team during his tenure but one big question ahead of Saturday's Six Nations opener is over who will partner Garry Ringrose at centre.

Robbie Henshaw is ruled out through injury, which leaves Farrell with the option of Bundee Aki, Stuart McCloskey or the uncapped Jamie Osborne.

O'Sullivan says he thinks it is a straight fight between Aki and McCloskey for the starting spot ahead of Osborne, who has impressed for Leinster.

Aki has struggled for minutes at Connacht while McCloskey was handed the staring jersey while his rival for the centre spot was suspended for the Autumn Nations Series.

"I don't see him going with Osborne to win his first cap away from home. I'd be very surprised," O'Sullivan added.

"Bundee Aki hasn't played in six weeks. Now, they've played Bundee before when he hasn't played much rugby but, on form, you have to pick McCloskey.

"I don't know which way he [Farrell] will go. I think he might go with Bundee Aki because he has done that before when he has been out for a while.

"At the same time, there is a risk in that because he has played very little rugby and I think the last time he played for Connacht was before Christmas."