The Aga Khan-owned colt had to overcome a morning injury scare on the way to Epsom glory earlier this month, but had no such problem with the short trip from his trainer Dermot Weld's stable at Irish Headquarters.
In the race itself, however, Harzand had to show all the qualities of a top-class thoroughbred as his old rival Idaho did not go down without a fight.
The leading pair pulled clear in the final quarter-mile, with Harzand, the 4-6 favourite, asserting close home in the hands of Pat Smullen to win by half a length.
Stellar Mass was three and a quarter lengths away in third and Ed Dunlop's British raider, the supplemented Red Verdon, just behind in fourth after putting in good late work.
Harzand's pacemaking stablemate Ebediyin did his job to ensure a good gallop and was allowed an easy lead. Idaho's stable companion Port Douglas followed him in second place, with Harzand hard on their heels.
Smullen decided to use Harzand's stamina to the full by taking the lead as soon as they turned for home.
But the Aidan O'Brien-trained Idaho, who had finished third behind Harzand at Epsom, soon ranged alongside under Ryan Moore and a thrilling duel ensued.
The two protagonists quickly put daylight between their rivals and there was nothing between them until Harzand proved the stronger close home to become the 18th horse to complete the Derby double.
Harzand is the fifth horse to win both the English and Irish Derbies for the Aga Khan after Shergar (1981), Shahrastani (1986), Kahyasi (1989) and Sinndar (2000).
It was Weld's third win in the Irish Derby after previous triumphs for Zagreb in 1996 and Grey Swallow in 2004, and a second for Smullen after riding the latter.
"I knew Aidan would have his horse better than at Epsom so I knew our horse had to be better than at Epsom to win again," said the Rosewell House handler.
"He ran a magnificent race. He showed the speed that he has today. People were saying he didn't have speed, well you saw it today.
"Those two horses really picked up and quickened. Idaho is a high-class horse, he's always been that and he was at his best today. It was a great day, a great ride - a special day. It was a proper horse race.
"He'll have a good break now and we'll train him for the Arc."
Weld had been somewhat guarded on his chance at the beginning of the week and added: "I was concerned because he had a very hard race at Epsom. We always knew he was very tough, but he had to be 110 per cent today.
"This is a very, very special race and I've been enjoying it since I was a child. I've been fortunate enough to win it three times now."
The Aga Khan said: "This horse is very good. It's a wonderful family and we checked him with all our mares and he'll mate very well with them, so we're looking forward to when he goes to stud and we'll be right behind him.
"Things are shaping up very well. I'm not sure we really know his limits. Neither at Epsom nor here, I don't think we've seen the best of this horse, he can do more."
Smullen said: "He's such a great horse, he's got such courage and determination. Ryan came with one good determined run, I needed this horse to dig deep for me and he did - how good is he?
"I don't think I've ever ridden such a courageous horse. I just can't get over his courage in the last half-furlong.
"This matches Epsom, in front of our home crowd. The pressure was on and we pulled it off. He's a dual Derby winner now.
"You can give horses good rides, bad rides - but when they're as good as this lad and they're as determined as this horse it just makes it easy. There's no knowing where this horse will end up.
"He's so tough and he's not just tough, he's got pace. He's a very, very good horse and I'd love to think he'd be an Arc horse."
Medicine Jack the lad for Railway Stakes
Medicine Jack put up a game display to outpoint Peace Envoy in the GAIN Railway Stakes at the Curragh.
The leading pair were separated across the track but though Peace Envoy had the rail, Medicine Jack galloped on resolutely in the centre of the course to come out on top.
The early pace was set by Mirdif but there were plenty in with chances in the final quarter-mile.
Peace Envoy looked to hold a narrow lead under Ryan Moore in a bid to give trainer Aidan O'Brien a 13th success in Group Two contest.
However, the Ger Lyons-trained Medicine Jack (6-1) found more for Colin Keane and got the verdict by half a length.
Dream Of Dreams, trained in North Yorkshire, by Kevin Ryan, was a length and a half away with third.
It was a welcome change of fortune for Keane, who was involved in a nasty-looking incident earlier on the card when his mount Elusive Heights was brought down.
Celebration time for Sruthan and Paul Deegan
Sruthan (10-1) denied Sovereign Debt back-to-back victories in the Dubai Duty Free Millenium Millionaire Celebration Stakes when getting up in the closing stages to snatch the Listed spoils at the Curragh.
Victory looked to be in Sovereign Debt's grasp when Chris Hayes sent the Dandy Nicholls-trained seven-year-old into the lead two furlongs out, after Lily's Rainbow and Brendan Brackan had set the pace.
However, Colm O'Donoghue conjured a late run out of Paul Deegan's six-year-old to lead 50 yards from home and win by three-quarters of a length. Hint Of A Tint was another neck away.
"That's great. Good horses change seasons," said Deegan.