What a performance. How courageously carved out, how beautifully executed, how perfectly timed.
In what was their last opportunity to get it right ahead of Saturday's Heineken Cup quarter-final against Saracens at Twickenham, Ulster returned to the form which had seen them sweep all before them in the first half of the season.
Then injuries to key players coupled with the loss of others to the Six Nations combined to take their toll, with one win in five RaboDirect PRO12 outings suggesting the bubble had burst.
In the circumstances, the challenge they faced could not have been more daunting – Leinster the champions of Europe in three of the past four seasons and PRO12 table-toppers going into Saturday night's inter-pro at the RDS where Ulster had never won a match.
Every bookmaker in the country made the hosts favourites. But Ulster made a nonsense of the odds and fools of those who had offered them.
Even so, captain Johann Muller proved his huge feet are firmly on the ground when he said: "Look, we haven't achieved anything yet. It was a great result, a great win, the first time in 13 years that we have shown we can actually beat them here in Dublin.
"But this is just one more small step in the right direction and we need to follow that up with another one next week."
Ulster took any number of steps in the right direction. Trailing 6-0 they got back to 6-5. Down 9-5, again they closed to within a point.
At half-time they were 12-8 in arrears. But at the start of the final quarter they nosed ahead for the first time and from then on to the most bizarre finish I have ever witnessed, referee George Clancy blowing for full-time only then to decide on a consultation with his TMO (video referee) as to whether Leinster might have scored what would have been a match-stealing try. Drama? That doesn't begin to describe it.
Ulster had stars and heroes in every position, none more than tight-head rookie Ricky Lutton who never flinched against Cian Healy, number one in Leinster's all-international front row. This was Lutton's first start as a senior; what an entrance.
Asked if he is looking forward to Twickenham and Saracens the modest 27-year-old replied: "There are two international tight-heads (John Afoa and Declan Fitzpatrick) in the squad above me so I'm not counting my chickens yet. I'll just turn up at training on Monday and go hard at it and see what happens."
Lauding Lutton's performance, coach Mark Anscombe said: "I thought he did superbly. Outstanding against an international, a world-class loose-head prop. He was nervous, but he had the faith and confidence of everyone around him and he acquitted himself superbly."
That was true of Ulster as a whole. They outscored their rivals by two tries to nil, with Ian Madigan landing six out of six penalties for Leinster, four of them in the first half.
A 22nd minute touchdown by Robbie Diack and a 39th minute penalty by Ruan Pienaar were Ulster's pre-interval responses.
A fifth Madigan penalty three minutes after the resumption left Ulster seven points in arrears, despite the fact they were playing all of the rugby, 15-man stuff featuring numerous phases, variety, strength and subtlety in almost perfect measures.
The exquisite Pienaar – kicking in preference to Paddy Jackson as one imagines he will be from now to the end of the season – responded with a gem from 45 metres to keep Ulster in touch at 15-11 and then obliged again, cutting the deficit to just a point with 23 minutes remaining.
A try by Iain Henderson – a replacement for Nick Williams – followed, with Andrew Trimble at the core of its creation. That gave the visitors a four-point lead.
A sixth Madigan penalty after 69 minutes saw Leinster close to 19-18, but Pienaar promptly replied in kind. And from there to the finish it was a case of wave upon wave of blue and tackle upon tackle by white-shirted heroes whose defence was stunning.
Anscombe wore the smile of a man whose players had answered those who doubted them. Since the start of the season he has been creating 'firsts' and here was another; first Dublin win since 1999, first-ever Ulster triumph at the RDS, first same-season home and away double over Leinster.
Putting the win in the context of the bigger picture he said: "From our point of view what was important tonight was the performance. It was great to get the win, but we needed to get some momentum going into next week.
"We're in two competitions and that's what every team wants at this time of year."