The result was never in doubt from the moment Wales opened the scoring after just 71 seconds – and no-one handled the atrocious conditions better than man of the match Gareth Anscombe.
As well as setting up the only try for Hadleigh Parkes with a delicate kick, Anscombe rounded up his own Magnificent Seven by kicking six penalties and a conversion for a twenty points haul.
That meant the big prize was secured with easily the most dominant performance of Wales’ all-conquering Six Nations campaign – and their 14th successive win also saw them leapfrog over Ireland into second place in the world rankings.
As well as adding to the 2008 and 2012 Grand Slams masterminded by departing coach Gatland, Wales were also celebrating their fourth Championship since he took over in 2008, although the 2013 success came when he was on Lions duty.
Gatland even earned a Royal thumbs-up from Prince William, who spoke of his admiration for the New Zealander-born coach in the match day programme.
The Welsh Rugby Union patron seemed to know what was in store, as he wrote: “I’d like to pay particular tribute to Warren Gatland.
“I think Warren has been sensational throughout his career as a coach, and it’s sad that it will be his last Six Nations with Wales. I’m sure successes with the Lions might be in the future, I hope.”
Ireland must also have been regretting their decision to keep the roof open, as the heavens opened just before kick off, as forecast.
For a team whose main attacking weapon is continuity – a million pick and drives and going through more phases than Benjamin Button – that decision seemed baffling.
Sure enough their handling errors meant they never built any momentum, and the best defence in world rugby was never seriously threatened until it no longer really mattered.
In their wildest dreams, Gatland and his players could not have imagined a better start than the one they made here, with that converted try stemming from their first attack.
Anscombe’s pinpoint kick off allowed Geroge North to force opposite number Jason Stockdale into touch, and referee Angus Gardner immediately signalled a penalty to Wales as they won the lineout and set up a rolling maul.
That left Anscombe with a kick to nothing – but his carefully-weighted chip over the top proved to a masterstroke, as Parkes clutched it out of the air and dived over the try line.
Anscombe converted and the Welsh fans broke into their first chorus of Hymns and Arias inside the first four minutes, surely a record. The singing never relented after that.
The home fans were almost silenced as a quickly-taken cross kick by Johnny Sexton sent Stockdale haring down the left hand touchline. He handed off Gareth Davies and looked certain to score, until that man Parkes felled him with a brilliant tackle.
It was all action, harem-scarem stuff in a breathless opening quarter, and Wales had to reshuffle after just nine minutes as they lost North with a hand injury.
Liam Williams switched to the wing and Anscombe went to full back, with Dan Biggar taking over at fly half.
Scrum half Davies was not having a great time of it, and when he dived in on Bundee Aki, it sparked a mass brawl and led to Gardner reversing a penalty to Wales.
That cost the home team a potential three points, but they got the next score anyway.
Anscombe defied the driving rain to land a monster fifty yard kick, and Wales were 10-0 ahead.
Ireland went for the corner after finally winning a penalty inside the Welsh 22, but the Dragons repulsed the green maul, and relieved the pressure when CJ Stander fumbled the ball while trying to take a quick free kick.
That showed how rattled last year’s Grand Slam winners were. Their much-praised lineout was struggling and Wales forced them to concede a string of penalties at the scrum.
Anscombe slotted two more superb kicks from a coyuple of those awards, and Wales headed in at half time with an imposing 16-0 lead.
The pessimists in the Cardiff crowd – and there were a few – pointed to the fact Wales were 16 points behind at half time in Paris before beating France 24-19.
But on the other side of the coin, Ireland had never managed to come back from more than eight points down to beat Wales – and Gatland’s players were in no mood to allow their opponents an historic triumph.
Anscombe added two more penalties, and Wales seemed furious at themselves when Larmour finally race over with the clock well into the red – but everything else that mattered was decked in red as well, including the Six Nations trophy.