To be a Leinster player or supporter this weekend is to be standing at an era-defining junction. One way lies the sweetest of dreams: a record-equalling fifth gold star on the blue jersey and Champions Cup immortality. The other leads straight to their worst nightmare: another painful knockout defeat at the hands of their all-too-familiar bogeyman.

Up until last week’s shock home loss to Munster in the semi-final of the United Rugby Championship, the former felt slightly more likely. Suddenly, though, they have played straight into the hands of their old friend Ronan O’Gara, the head coach of La Rochelle. If ever “Rog” needed a handy pre-match omen, it is his old province winning on the same stretch of grass last Saturday.

Never mind the slightly weakened side that Leinster fielded with half an eye on the looming French challenge. Give O’Gara an inch mentally and he will take a country mile. Having already steered La Rochelle to victory in last year’s final against the same opponents in Marseille, the former Ireland out-half – “it’s a home game for me too” – can exploit a psychological edge like few others.

Because O’Gara knows precisely how much Leinster will hate seeing another trophy slip away. Last season they went down to La Rochelle by three points and the Bulls by a point in the URC semi-final. Now, courtesy of Munster, they have just lost another huge game by a point. Is it a trend? If it is still close entering the final quarter – and there is every reason to imagine it will be – there is no doubt which team will start to feel the most creeping pressure.

In such circumstances, O’Gara will be telling his players, La Rochelle have already shown their mettle. Last year he urged them to be brave in thought and deed if they wanted to secure the club’s first Champions Cup. They duly went out and outscored Leinster three tries to nil, the last of them in the 79th minute from replacement scrum-half Arthur Retière, to secure a thrilling 24-21 victory. “We knew we’re a second-half team,” said O’Gara afterwards. “We knew that the last 20 minutes was where we could get them.”

It was La Rochelle who also steamrollered Leinster 32-23 in the 2021 semi-final. A coincidence? Hardly. O’Gara knows Leinster every bit as well as they know themselves. He understands how they like to play quick and slick, particularly off first phase, as they were allowed to do in the quarter-finals when they blasted Leicester clean off the park. He knows they have the ability to strike from deep, particularly with James Lowe back to give them a little X-factor out wide. He knows they also like to apply scoreboard pressure and make their opponents sweat that way.

La Rochelle’s Grégory Alldritt lifts the trophy with his teammates after the Champions Cup final victory over Leinster at the Stade Vélodrome in Marseille last year
La Rochelle’s Grégory Alldritt lifts the trophy with his teammates after the Champions Cup final victory over Leinster at the Stade Vélodrome in Marseille last year. Photograph: David Davies/PA

Which is why La Rochelle will be focused on denying their opponents any such luxuries. As Exeter found out in their semi-final in Bordeaux, the defending champions’ power game is far from their only strength. Probing other people’s weaknesses is another speciality. “Just because an opponent knows what’s coming doesn’t necessarily mean they can stop it,” O’Gara suggested, slightly provocatively, this time last year. And sure enough, when the moment of truth arrived, Leinster found it harder to bring their own obvious talents to bear.

It could be argued, too, that La Rochelle are more formidable now than a year ago. For a start they have a fit Tawera Kerr-Barlow, who has scored five tries in seven games during this European campaign, back to do hyphenated battle with his old Kiwi mate Jamison Gibson-Park. Kerr-Barlow is potentially eligible for Australia at this World Cup despite having previously won 29 All Black caps. If another Wallaby, the giant Will Skelton, is lacking in any area, it is a brave man who cranes his neck skywards to tell him so to his face. No one could possibly claim, either, that Uini Atonio lacks set-piece ballast. Leinster like to blitz teams but few have the artillery at O’Gara’s disposal.

All of which adds up to a potentially gripping encounter. Leinster are without the masterful Johnny Sexton and, despite Ross Byrne’s continuing improvement, that removes some of their swagger. Their back-row kings, Josh van der Flier and Caelan Doris, usually rule the roost but they do not have to deal with Levani Botia and Grégory Alldritt every week. France may have lost in Dublin this season, as did Toulouse in the semi-final, but the men from the Atlantic coast will take the field with genuine belief.

The outcome will be keenly monitored from afar. In England they will be wondering how on earth, in the short term, they can compete with the vast renewable energy generated by the best modern Irish and French sides. In the southern hemisphere they will be looking for clues – maybe weariness, perhaps a degree of predictability – that might offer distant hope ahead of this autumn’s onrushing Rugby World Cup.

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Nowhere, though, will it feel more real than in the quadrant of Ireland that has come to recognise itself as all-conquering. Leo Cullen, ably abetted by the soon-to-depart Stuart Lancaster, has done wonders in terms of maximising the rich playing resources at their disposal. Under their joint stewardship good players have become better ones and their level of consistency is universally admired.

Tadhg Furlong and James Lowe during a Leinster training session in Dublin this week
Tadhg Furlong and James Lowe during a Leinster training session in Dublin this week. Photograph: Harry Murphy/Sportsfile/Getty Images

But the ledger also shows Leinster have won the ultimate prize just once in the past 10 seasons – and that edgy, low-scoring affair against Racing 92 in Bilbao five years ago. Since then, on their two subsequent final appearances, they have been steamrollered by Saracens (and Skelton) in Newcastle and heartbroken in the Stade Vélodrome this time last year. To stumble again, with a record-equalling fifth crown still tantalisingly out of reach, would be tough to bear and trigger some serious soul searching.

The question, then, is whether their sheer desire not to be caught short again – allied to the support of the resident blue army at this most familiar of venues – will be enough. Or are we about to see another chapter written in O’Gara’s remarkable top-level career, both as a player and a coach? Clearly there is a bigger picture involved here, not least two squads brimming with quality at a time in the game’s history when everyone could really do with witnessing an irresistibly compelling advert for the sport. The biggest battle from Leinster’s perspective, though, has become a mental one. And in that kind of head-to-head duel, it rarely pays to underestimate their old Munster-reared adversary.