Back in the day Ireland team manager Pa Whelan used to have a 'ready to go' line whenever he was asked about a player who had been left out of the team. "We know what he can do," he would say about the poor unfortunate who didn't make the cut. The subtext was that Whelan and Co knew better what the player in question could not do.

That line presented itself again last November when Simon Easterby dialled up his Pa impression on the subject of Seán Cronin. Joe Schmidt had just announced his squad for the three-game Guinness Series, and the hooker was surplus to requirements.

"We know what he is about, know how good he can be, know his strengths," Easterby said at the time. "We felt at this stage it was a good time to invest some time in others. That might change in the Six Nations."

The last line seemed to leave the door more than ajar. But it wasn't as if Cronin had been told a little nudge would open it .

"No," he says. "I was told, 'Look, other guys are playing well', which they were at the time. I wasn't playing well. They never patted me on the back and said, 'We'll make sure you get back in'. I obviously knew that I had to go away and work hard. I like to think that's the kind of person I am. That gave me the kick that I needed to address the stuff I needed to address and get right."

So he got the Pro14 games he needed with Leinster to get back on track, build some momentum, and then get stuck in again to the Champions Cup. Currently that head of steam has Cronin going to Bilbao next weekend in the form of his life, which is a good place to be for a man who first turned heads in an AIL final in 2006.

He readily accepts that the way he started the season was well removed from ideal: he was carrying extra weight and struggling to come back from injury. In truth, for Seán Cronin the build-up to the announcement of the November squad was more about resignation to what was coming than surprise that it came.

"I watch a lot of rugby," he says. "I'm no fool. I saw Rob Herring had been playing really well, I think Rory (Best) was out injured at the time and he was getting a lot of game-time and playing really, really well. James Tracy was playing better than me here (at Leinster) so sometimes you've just got to acknowledge it. If your form has dipped you've got to address it, and maybe other guys deserved a chance at the time. I'll be 32 on Sunday and that kind of stuff comes with age. As you progress through your career, you learn from stuff."

One of the first lessons he learned was the numbers game. As a young star on Shannon's three-in-a-row AIL side of 2004-2006, a place in the Munster set-up was the logical companion to a developing career. It was a crowded house though. And after three seasons his tally of pro games numbered two.

"I don't want to give you an answer that's easy to say, but it's such a personal decision. For me, yeah, it was tough to leave Munster but I was fourth- or fifth-choice hooker and Declan Kidney was bringing in guys in front of me even when there were injuries, which kind of annoyed me a small bit.

"I thought I was good enough to be at least third-choice there, and if there were guys away on international duty to be on the bench, but it never came to pass so. It was tough for me to leave. You know I admire guys who are one-club men - it's a good thing to have in your career to look back on. But I also admire guys who have the balls to move and are willing to be, not ambitious, but if they can see it's a better career path."

Connacht gave him what he needed: the chance to play. When he had proved himself good enough to move east then that's what he did. Already part of the Ireland set-up by then, he arrived in Leinster looking first to learn from Richardt Strauss - about to get a bit of green himself - and then overtake him.

That manoeuvre was complete by 2013/'14 by which time he had started 18 of his 23 games that season for Leinster. The Ireland bit has never quite caught on though. Cronin's haul of 61 caps is 60 more than most in the game. But if you offered him the chance to move those numbers around the page a bit then surely he would want more than nine to have been starts.

Whether it was Jerry Flannery or Rory Best, the role for Cronin has mostly been as glamorous assistant. Physically bigger than either man, and with enough gas to burn off pretty much any rival at any stage in his career, Cronin has had his issues with darts. And it's hard to find those who would describe him as a scrummaging hooker. In which case he has been packaged as the epitome of impact. With outstanding props under his oxters in Leinster, and second-rows who know their job, he has looked the ideal all-rounder as this season wore on.

"I looked to have a level of more consistency around my play in general this season, so I could bring the whole package together," he says. "In terms of that, I think I'm probably performing to the best of my career in terms of my scrummaging; my lineout has been a lot better this year, maybe in the last 18 months. They're areas of my game I've tried to work hard at, both here and with Ireland, to try and get better so I could play at the top, top level."

Putting the frighteners on Bundee Aki in open country in the RDS in January illustrated how his progress was unfolding.

"I suppose I took a lot of confidence from those back-to-back (Champions Cup) games. The whole squad took a lot of confidence from going away from home and putting in a clinical performance against Exeter and then building into the Christmas games. Bundee? I hadn't seen his eyes light up like that in a while when I tried to take him on the outside! He told me afterwards: 'Don't ever do that to me again!' Yeah, it was good, like I said, I felt good at the time. I felt fit. I was working at 105 or 106 kilos at the start of the year. It was great for the scrummaging but I've got back down to around 100/101 and I think even the work I've put in with the scrummaging, I feel the loss of weight hasn't really affected me and I'm moving a lot better on the ground and it just makes a big difference with me."

Two things present themselves in Bilbao on Sunday. The first is Donnacha Ryan and the prospect of him doing to Leinster when he did to Munster in the Champions Cup semi-final. Racing's defensive lineout is outstanding, and leading it is a badge of honour for Ryan.

"I know Donnacha really well," Cronin says. "I played at Shannon with him years and years ago - that's how long we go back. All he does is trawl through video, even with Ireland, it was video, video, video. He's like Tom Brady in the NFL so he'll be well-versed in what we're doing so we'll probably have to adjust a few things for him."

Then there is the intensity of the occasion itself. In 2012 in Twickenham, Cronin was coming off the bench for Strauss and running in a try as part of Leinster's final jamboree against Ulster, their third title in four seasons. The magnificent semi-final contest against Bordeaux seems like a more useful pointer to Saturday in Spain.

"I've actually watched that game on YouTube a couple of times," Cronin says. "The last 10 minutes of that Bordeaux game is a good reference point to go to where you can see the intensity. It was huge. The centre (Wesley) Fofana knocked the ball on going over the line and then we had two scrums on our line. And then there was about seven or eight minutes' defending, so that's a pretty good reference point to go to for the intensity we're going to have in Bilbao.

"We don't know what kind of crowd but we're hoping Leinster will have a huge following going there, but it (Bordeaux) was a great atmosphere to play in. I think Seánie O'Brien got the turnover on the line. It was hugely satisfying to go over there and put in a performance like that and get to another final."

Which is where he is again. This time as a starter. And as Pa Whelan might have put it: we know what he can do.

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