It was an eventful race early as Peregrine Falcon broke well from trap four and edged inwards while second-favourite Lenson Rocky, drawn against the fence, moved to his right.
Sandwiched in the middle were Cloran Paddy and Droopys Roddick, while Hiya Butt took his customary position at the head of affairs following a typically flashy start.
The eventual winner, owned by John Turner, also broke well and held a prominent position to the bend, but conceded ground around the turns as the leader cemented his position, with Lenson Rocky going in chase.
The raw speed of Droopys Roddick and Jaytee Jet shone through as they raced down the back and, by the time they negotiated the final turns, there were virtually four in line.
Jaytee Jet, racing widest of all, and Droopys Roddick, trying to negotiate a passage between runners, powered down the straight and, in a tight finish, it was 15-8 favourite Jaytee Jet who prevailed by a neck, from Droopys Roddick, with Hiya Butt running his heart to be just a short head further back.
In victory, Hennessy joined Nick Savva as the only trainers to have won the English, Irish and Scottish Derbies.
“He’s a brilliant greyhound - what a performance from a young greyhound on a fast track,” the emotional winning trainer told Sky Sports. “They said if the track was heavy and slower it would suit our guy, but I said to John (Turner) beforehand ‘this dog has more speed than they’re giving him credit for’.
“He’s an incredible competitor, and thanks be to God it worked out. I thought I’d never win the race.
“It’s great for the team – I have a wonderful team. And I have a lot of very special people looking down on us tonight, and I have no doubt they helped to get that dog over the line.”
In the action at home, there was a huge upset in the first semi-final of the Sporting Press Irish Oaks at Shelbourne Park when Ballydoyle exited the competition, finishing fifth behind runaway winner Holycross Leah.
The latter, trained by Jerry Melia, produced a career-best break, and that allowed her to dominate from start to finish.
The 6-1 chance maintained a strong gallop to beat Leamaneigh Mags by four and a half lengths, in 28.21, with Ballymac Sarahjo denying Leezie Lindsay a place in next week’s decider.
Ballydoyle Honey clearly wasn’t herself and, after looking all at sea throughout, failed to pick up with her customary style.
Sidarian Pearl made the most of her inside draw when taking the second semi-final for trainer Graham Holland. The 1-2 favourite flashed out of the inside box and had no trouble leading kennel companion Rural Maui to the turn. Never in a moment’s danger, she stayed on well to beat the fast-finishing Witches Belle by two lengths, in 28.38, with High St Jesse in third.
1. High St Jesse, 2. Sidarian Pearl, 3. Ballymac Sarahjo, 4. Holycross Leah, 5. Leamaneigh Mags (m), 6. Witches Belle (m).