The Leinster head coach had seen his Champions Cup winners stretched to the limit by their arch enemies in Saturday’s pulsating encounter at the RDS and just about come out on the right side of the contest.
Hanging onto their one-point lead at the final whistle allows Leinster to keep alive hopes of a first European- Celtic League double, with the PRO14 final on Saturday against defending champions Scarlets at the Aviva Stadium.
It was a triumph for Cullen’s team, for the depth of character of his squad to back up in a tough interprovincial knockout game just seven days after that nail-biting war of attrition against Racing 92 in Bilbao and to beat a Munster side desperate for their own first piece of silverware since 2011 without a number of frontline internationals and a captain, Isa Nacewa, playing on one leg until succumbing at half-time.
Yet as Cullen’s evident relief attested, Munster will have returned south on Saturday night knowing they only have themselves to blame for letting Leinster off the hook in a contest they outscored their hosts by two tries to one but lacked the composure and discipline to finish the job.
While Munster head coach Johann van Graan and captain Peter O’Mahony pored over the debris of a second semi-final defeat in four weeks, Cullen might have been the man who survived the firing squad, although for long stretches of this game it was the men with the rifles who appeared to be wearing the blindfolds.
Van Graan’s squad had badly needed to make a statement in Dublin.
A dismal opening half-hour against Racing had condemned Munster to also-rans by half-time in last month’s Champions Cup semi-final.
There followed some strait-jacketed home performances to draw with Ulster and then edge past Edinburgh in the PRO14 quarter-finals,
And to a degree they did, starting with intent on a glorious day in D4, only to have their pockets picked by man of the match James Lowe, whose brilliant offload to Jack Conan gave Leinster the opening try on seven minutes.
Converted by Joey Carbery for a 7-0 lead there was further woe three minutes later when Munster’s plans to stifle Leinster at the breakdown where hit by a serious knee injury to openside flanker Jack O’Donoghue.
Munster continued to take the game to Leinster only to let themselves down with poor execution, their pressure undone by sloppy handling, inaccurate kicking, forward passes and a yellow card for Jean Kleyn when they threatened most on the five-metre line.
It was a clumsy, wreckless clearout of fly-half Ross Byrne that left South African referee Stuart Berry with no option but to sin bin his compatriot four minutes before the break, allowing Leinster to take a 10-3 lead into half-time.
Munster started the second period brightly, this time inflicting the wounds on Leinster rather than themselves as Keith Earls hustled into the corner three minutes after the restart.
Ian Keatley, on at the interval for the injured JJ Hanrahan, missed the touchline conversion but when Kleyn returned Munster had won their short-handed 10 minutes by five points to three, Byrne, cleared from his Kleyn-applied HIA, making it 13-8 with a penalty just before the lock reappeared.
Byrne would miss another on 51 minutes, Carbery following suit shortly after as this attritional contest continued to Cullen’s frustration but yet again Munster handed Leinster a get out of jail card.
Winning a penalty inside the opposition half and offering a long-range shot at goal with 11 minutes remaining in a five-point game, O’Mahony pointed to the corner and instead of aiming at the posts Conor Murray fired the ball into the left corner.
It was a ballsy move from the Munster skipper but when the lineout came under pressure the visiting pack’s planned drive was scuppered and Murray moved the ball along the line, meeting strong resistance in blue at every contact point until in the opposite corner replacement hooker James Tracy clamped onto the ball and earned his side a precious, pressure-relieving penalty.
Another Leinster penalty would follow, the pragmatic view taken as Carbery bisected the posts to open up a 16-8 lead with four minutes to go and though replacement Gerbrandt Grobler got the ball to the posts, allowing Keatley a simple conversion to narrow the gap to a point, it would prove yet another case of close but no cigar for Munster and another final for Leinster.
“I think we showed some really good characteristics in the first-half,” Cullen said.
“Munster did well, incredibly well, at the start of the second half. When they were down to 14 men they scored that try.
“They did incredibly well. I’m not sure are we just relying on ‘okay, well, we’ve got the sun on our backs and the breeze on our backs and we’re just going to play down there because we’ve got a bit of a lead.’
“Suddenly then it just becomes this unbelievably tense affair. How we managed that could be better but ultimately we’ve won the game and we’re in the final so we just need to look forward now.”
For Munster in the immediate aftermath of defeat there was only looking back at what might have been but O’Mahony stood by his decision to go for the corner in that crucial moment with 11 minutes to go.
“That’s my call. I have to make those calls,” the captain said. “I felt like we had a lot of momentum. We were winning a lot of collisions. We were going well up the field. That was my decision at the time, that’s the way I felt it was going.
“Look, if we had gone there and scored, it would have been a big momentum changer obviously, but we were inaccurate at the lineout.”
While Munster are consigned to licking their wounds, Cullen now looks forward to trying to achieve a feat he never managed as Leinster captain: adding a domestic title to a European trophy.
“It’s hard for us to think like that (about a double) at the moment because you have to figure out how to go about winning the game,” the Leinster boss said. “How do we go about giving a better account of ourselves?
“There were a lot of things from even that game which we could have done better. We squandered a couple of opportunities. It gives us something to work on.
“We’ve something to review. We’ve something to look forward to. There are things we will take out of when we played Scarlets last in the Aviva (in their Champions Cup semi-final). I’m sure they will take plenty as well. It is just making sure we put in a good plan in place, make sure we have a chance of going on to win the game.”
J Carbery; J Larmour, G Ringrose, I Nacewa - captain (R O’Loughlin, h-t), J Lowe; R Byrne (B Daly, 36-40), L McGrath (N McCarthy, 77); J McGrath (C Healy, 46), S Cronin (J Tracy, 56), T Furlong (A Porter, 56); D Toner, J Ryan; R Ruddock (S Fardy, 62), Jordi Murphy (M Deegan, 79), J Conan.
S Zebo; A Conway, S Arnold (D Sweetnam, 70), R Scannell, K Earls; JJ Hanrahan (I Keatley, h-t), C Murray; J Cronin (D Kilcoyne, 53), R Marshall (N Scannell, 59), J Ryan (C Parker, 73); J Kleyn, B Holland (G Grobler, 51); P O’Mahony - captain, J O’Donoghue (R Copeland, 10), CJ Stander.
Kleyn 36-46 mins
Stuart Berry (South Africa)