VIDEO: Sexton savours Irish Six Nations title win
VIDEO: Sexton savours Irish Six Nations title winSHARE
The Irish were assured of it when defending champions England lost 16-22 to France in Paris.
The Irish – who can win only their third Five/Six Nations Grand Slam if they beat England next Saturday – had earlier secured a bonus point win over Scotland in Dublin beating them 28-8.
The Irish are on 19 points and England and France on 10 going into the final round of matches.
Sexton said it was a strange feeling to have already sealed the title with the England game still to come.
"It is a very strange feeling to win the championship with a game to go," said Sexton.
Sexton, who converted all four of Ireland's tries, said it was a huge achievement given serious injuries ruled out several key players during the campaign such as flank Josh van der Flier and centres Robbie Henshaw and Chris Farrell.
"Credit to the management team to have prepared us so well for each match with so many players missing," said Sexton.
"Beforehand if someone told me you are going to win the championship without van der Flier, Heaslip and Henshaw I wouldn't have believed them.
"It is great we have built a bit of depth but we know the biggest challenge [England] lies ahead."
Schmidt for his part said the title should be enjoyed by all those who had missed out either through injury or just not getting game time.
"From a wider perspective the players out there have won this title for the injured players and those who worked really hard but didn't get to play," said Schmidt.
The Irish began their campaign with a nerve-wracking last-second win over France and have secured three successive bonus-point wins – for scoring at least four tries – since, all at Lansdowne Road over Italy, Wales and then the Scots.
Sexton, who along with scrumhalf Conor Murray has pulled the strings for the Irish, said his drop goal at the death against the French may have been the defining moment of the campaign.
"That was a very special moment for the team look," said Sexton.
"We will look back on it when we lift the trophy next weekend and say that was a huge moment," added the 32-year-old.
Schmidt took over a demoralised Irish side after the 2013 Six Nations but then guided the Irish to back to back titles in 2014/15.
England won the subsequent two – the Irish denying them successive Grand Slams by beating them in Dublin last year.
The English will be keen to return the favour at Twickenham – where they are yet to taste defeat under Eddie Jones since he took over after the 2015 World Cup debacle.
"As regards the England game the shoe is on the other foot after last year," said Sexton.
"They will be extra motivated and knowing many from the Lions tour they are a great bunch and will be gunning for us."