THERE comes a time when the pupil comes up against his old master and the pupil wins. Could it be that one of those is coming on Saturday when Glasgow Warriors take on Leinster at sold-out Scotstoun?

Apart from the catalogue of issues surrounding the teams and the comparative states of Scottish and Irish rugby, there is also a fascinating side issue in the battle of the bosses where Leinster’s Leo Cullen is up against Dave Rennie, the first man he turned to when he wanted to make the transition from club captain in Dublin to coach.

Rennie was then at the Chiefs in New Zealand, and though Cullen was in his final season as a player, he was anxious to start work on the next stage of his career. “I took a trip to New Zealand and he was one of the people I sought out,” Cullen recalled. “It was during the Six Nations window, when I had a bit of time to travel. The Chiefs were a team I had admired from a very far distance. I wanted to see their set-up, the ethos of their team, their culture and what they were doing in coaching terms.

“Wayne Smith [who is back with the All Blacks] and Tom Coventry [who moved to London Irish] were there as well – the two of them were people I was also keen to talk to.

“Some clubs are very open with information and they were unbelievably good. I owe a lot to Dave Rennie. It was a great experience for me before I went into coaching. I met Dave again at the PRO14 launch a few weeks ago, he is a great coach and a good man. Glasgow have done incredibly well to get him.”

The personal battle between the coaches has to be seen in the context of a wider rivalry between the clubs. Cullen remembers losing to Glasgow as a young player, but the Irish province soon overtook the Scots, becoming three-time European champions, six times PRO12 finalists and twice winners, including in 2014, Cullen’s last season as a player, when they beat Glasgow in the final.

Of late, however, games between them have been nip and tuck, and neither can realistically claim to be favourite this weekend, even though Glasgow did win their last Scotstoun match 33-25.

“There’s a great atmosphere there, with what has been created over the last few years,” Cullen said. “You can see the way that everyone is involved now – it’s great.”

Cullen can see Rennie’s influence, building on the foundations laid down by Gregor Townsend. “Probably around the ruck, in how aggressive they are around the clean-out,” was his assessment of the biggest change.

“The way their backs work off the ball is impressive to watch. Even their set-piece is impressive and they have a very strong driving game. In all the fundamentals of the game they look like they’re putting plenty of work in.

“Everyone wants to be the best team in Europe. You take that as a given. There was a time when we might have been the best team, but the landscape is so different now.

“In terms of coaches and staff, Glasgow are probably better resourced than most teams. They have recruited good coaches. They have improved gradually. That’s the way you have to do it – bit by bit. Leinster went through a similar cycle themselves. Then you get to the summit and then people are trying to knock you off. The game is changing all the time.”

For all their success, Cullen has every reason to think he has a few points to prove against his old mentor. He took over when Leinster were PRO12 champions but in his first season they failed to reach the play-offs. They last won the European crown in 2012 – since then they have lost two semi finals, one quarter final and twice failed to reach the knockout stage.

Cullen added: “It has been a reasonably rapid rise I suppose for me; I’m not sure if that’s a good or bad thing!”