Paris (AFP) - Ireland winning the Six Nations title will not have a direct impact on their hopes of finally reaching at least the Rugby World Cup semi-finals, but the key to both is Johnny Sexton remaining fit, according to former Irish fullback Hugo MacNeill.
Sexton will be 38 come the World Cup in France in September, but the Ireland skipper is as pivotal to his team's fortunes as he was in the 2015 and 2019 editions.
When Sexton took over at fly-half from Ronan O'Gara, he had earned it and it was a natural succession.
And when not injured he has retained his world class form -- he was world player of the year in 2018 -- and those who were tried in his absence have failed to dislodge him.
Sexton inspired the Irish to winning the Six Nations Triple Crown and an historic series win in New Zealand last year.
As a result they go into their opening Six Nations match with Wales on Saturday ranked world number one.
Joey Carbery was seen as Sexton's likeliest successor.
So much so he moved to Munster in 2018 from Sexton's province Leinster but to general surprise the New Zealand-born playmaker has been omitted from the Six Nations squad.
"The Irish have more strength in depth in most positions than four years ago," MacNeill told AFP in an interview.
"However, the biggest issue regarding the Six Nations and the World Cup is Johnny Sexton.
"Ireland are very dependent on Johnny both as a player and as a leader.Johnny is crucial to the team.
"It is a tough one for Joey Carbery but he just did not nail it down as he needed to.
"The problem is nobody apart from Johnny Sexton controls a game like he does."
'They have won nothing yet'
The 37-time capped MacNeill -- a member of the 1982 and 1985 Triple Crown-winning teams -- says the hurt felt by the Leinster players in their defeat by La Rochelle in last year's Champions Cup final served to motivate Ireland.
However, he adds that although this is good mentally he is more concerned by the potential physical toll the Six Nations can take.
"Of course it is a benefit to have a good Six Nations to establish a pattern of victories," he said.
"The key though is not losing too many players for the World Cup especially in pivotal positions due to injuries in the tournament.
"The Six Nations is gruelling and attritional."
MacNeill, however, believes that under head coach Andy Farrell the Irish will not get ahead of themselves if they win the title.
Four years ago there were already ominous signs for the Irish who had a disappointing Six Nations a year after achieving the Grand Slam under Joe Schmidt.
They never recovered their vim and were outclassed by New Zealand in the World Cup quarter-finals.
"I think Ireland's senses tell them a good tournament does not mean a good World Cup," said MacNeill.
"They are a very different dynamic.Matches come thick and fast in a World Cup.
"I would fancy Ireland to beat France at home in the Six Nations but then the opposite to happen should they play France on their home turf in the World Cup.
"They won't get carried away even if they won the Grand Slam.That is fine but then for a World Cup you throw in to the mix New Zealand, Australia and South Africa."
MacNeill -- who also played three Tests for the British and Irish Lions on the 1983 tour of New Zealand -- says that in Farrell the Irish have the ideal man to steer them through their challenges.
"He epitomises the best of Lancashire," said the 64-year-old.
"He is a man of enormous integrity and no ego, the players listen to him and have massive respect for him.
"He had a lot of challenges crossing over to union with his rugby league background and it is great he has done so well.
"However, he would be the first to say they have won nothing yet as in the Six Nations title."