Iain Henderson sits down at the top table in the Kingspan Stadium's media room which he has requisitioned for our interview and admits he has never been able to sit quite the same since undergoing surgery on the badly torn hamstring he sustained last December in this same ground.

It seemed an almost apt scar, mental as well as physical, given it sidelined him for four months and made him a bystander for the Six Nations when he should have been one of its main acts.

“I wasn’t able to sit down properly for about three months afterwards, and had to sit to one side. Even now I sit slightly to one side, because it was a habit I got into for so long, because of the way the hamstring was torn off the bone – I think it’s called the Ischial Tuberosity – which is effectively your seat bone.”

About the only plus was two new Latin words, but even then it got worse before it got better.

“That was all sore and bruised,” he says of the aforementioned Ischial Tuberosity, “and I know this might sound like a bit of a cliché but on the flight home there was a kid sitting behind me kicking the back of my chair repeatedly. I was thinking ‘this could not get any worse’. But at least my mum came over with me and flew back home with me, so then I just laid up for a bit.”

Cue longs hours of physio and long hours in the gym. The episode seemed to sum up Henderson’s slightly cursed season. If he was to describe it in one word, the one that comes to his mind is “frustrating”.

By rights, it should have been something of a breakthrough year for the 24-year-old. Although already his fourth season in Test rugby, having made his debut at 20 in the 2012 November Tests against South Africa, this was going to be Henderson’s time to step up with Paul O’Connell leaving a huge void after his post-World Cup retirement.

Ironically, Henderson’s best of times with Ireland happened alongside O’Connell. Having played in both of the warm-up games against Wales, he broke into the Irish starting second row ahead of Devin Toner for the World Cup pool wins over Canada and Italy, before being demoted to the bench for the pool decider against France.

"I'm claiming that was tactical by Joe," says the typically easygoing Ulster lock.

Even then, he replaced the stricken O’Connell at half-time in that win over France. “You could see something serious had happened to Paul. It would have to be,” says Henderson, for O’Connell to be reluctantly removed from the fray for the last time.

“I knew I was coming on anyway. Joe had told me I was probably coming on shortly after half-time so I was already in the zone. When you’re on the bench for Joe you have to be ready to go on at any stage. So it wasn’t like I wasn’t expecting to go on.”

“In the changing room I just had a minute or two to gather my thoughts, to get myself right for the kick-off and then play half a game of rugby. I thoroughly enjoyed it. It was a fantastic win. Unbelievable Irish crowd and atmosphere. It was a great day,” recalls Henderson of that 24-9 win.

Cue those dastardly Pumas in the quarter-final a week later, when Henderson started in O’Connell’s absence.

“Going into the French game we’d yet to suffer an injury, but then everything struck us going into the Argentina game. It was unfortunate, the injuries dipped our performance and Argentina were all guns blazing.”

Purple patch

“There’s something about Argentina and Ireland in World Cups; they seem to get on top of us. They came out and played a real wide-to-wide pattern, and caught us on the edges, and we were 17-0 down at an early stage in the game. We did well, and were just three points down with a quarter of the game to go, but we had gone through our purple patch getting those points back, and the side Argentina were – they were playing good rugby – they weren’t going to go without scoring again.”

Even so, it seemed as if the baton had been passed on, just a little ahead of schedule. With such agility and footwork as well as his athleticism, Henderson is a rarity for an Irish lock. However, he had dislocated his finger against Argentina, which required surgery and then, in just his third game with Ulster after the World Cup early last December at the Kingspan Stadium against Edinburgh, he tore the hamstring off his bone.

“It was excruciatingly painful for about 30 seconds, and afterwards nothing more,” recalls Henderson with an ironic smile. “I was walking around the changing room and it felt fine. My dad came into the medical room, I walked over to him and he said: ‘You better not have come off that pitch with nothing wrong with you’. But I was worried and when I went for the scan on the Monday morning they told me the hamstring had come off the bone.”

Henderson went to London for his operation, ironically with the same consultant orthopaedic and trauma surgeon, Professor Fares Haddad, who had operated on O’Connell and on Premier Division footballers.

He’s now in bonus territory, in that the original prognosis was that Henderson would not return to action this season. Even so, it wasn’t easy watching the Six Nations.

“When you’re not playing and the team is doing well it’s really frustrating, because you want to be a part of it, but when they’re not doing so well you think ‘well flip, I could add a wee bit extra’ because in the first two games we were only three points away from two wins and then it’s a whole different scenario. Ireland’s tails would have been up again.”

Largest critic

Henderson watched the games from his home in Lisburn, which is near to where his parents Gordon and Zoe live. His dad was a keen number eight with the Academy club in his day, where his brothers Richard and Stuart also play, and has coached Henderson since he was seven.

“He’s been to all my games and has been my largest critic I would say,” says Henderson, smiling. He recalls his dad driving him to Newforge at 5.45am for his morning gym work and weight-lifting before school.

“It was ridiculous,” says Henderson. “I went in there before all the [Ulster] academy boys went training, who went in before the senior boys. It was like an elongated garden shed, made of tin, and it was freezing. My dad would sit in the car, waiting for me to finish, and then dropped me into school.”

Another coach was Stuart Olding’s dad, as the two boys have been classmates and buddies ever since their days together in Ben Madigan primary school.

“I loved school. School was one of the best times of my life,” he says of his secondary school days in Belfast Royal Academy. “I still go there to help out. I know all the coaches and I would still be friendly with some of the teachers.”

Henderson managed to play a few games with the school’s feeder club, Academy, alongside his brother Richard, before playing with Queen’s University in the Ulster Bank League, which he describes as a glorified extension of school, and with a similar freedom to play rugby. He studied economics and finance, but didn’t take to it, and has switched to pure mathematics in Open University.

Although his BRA team reached the Ulster Schools Senior Cup final, there was no Ulster or Irish schools recognition.

“Even when I played for the Irish Under-20s I wasn’t sure I could make it as a professional rugby player, but then my second season of Irish Under-20s was really enjoyable and I did well, got my first cap for Ulster and thought: ‘I might be able to do something here’.”

The 2011 Junior World Championships in Italy were, as he puts it, a disaster, as Ireland lost to South Africa (twice), England and Wales, beating only Scotland, to finish eighth with a team studded with provincial players such as Craig Gilroy, Andrew Conway, Brendan Macken, JJ Hanrahan, Eoin McKeon and Jordi Murphy.

“On paper it was the best U-20s team in years, a load of us had already played Magners League rugby, as it was at the time, but we just didn’t click. The following year, only one or two of us had played for our senior side but we beat South Africa (the hosts), Italy, England and France,” he recalls, as Ireland finished fifth.

Full academy

Henderson had been part of the Ulster/Leinster combination that thrashed their Munster/Connacht counterparts in the first game to be played at the redeveloped and reopened Aviva Stadium in May 2010. “I still have the jersey in the house.”

By then he was in the Ulster sub-academy, progressing to the full academy after a year and to the senior squad in 2012-13, but little did he think he would be back in the Aviva Stadium in November 2012 to make his Irish debut off the bench against South Africa. Henderson also started the win over Fiji in Thomond Park a week later, and appeared off the bench another week on in the win over Argentina.

His opposing number 20 on his debut was Marcell Coetzee, who is joining Ulster next season from the Sharks. Henderson sat beside Donncha O’Callaghan, who wore 19, that day, stood alongside him for the anthems and came on at the same time.

“He’s quite a character. When he ran onto the pitch I couldn’t keep up with him. Ruan [Pienaar] was also playing and I had a good chat with him afterwards. It was a fantastic day and all my family were there.”

“Even earlier that week, I thought I had just been called into the Irish camp to gain experience, almost like an academy player. I remember sitting there when Declan [Kidney] read out the team. I drift off at the best of times, but it came as such a shock to hear my name.”

Good teams

From that day against the Springboks, and for each of his 23 caps along the way, he likes to think of as many people as he can who helped him along the way, his father, his mum, his older brothers.

“It wouldn’t have happened without them, playing with and against them, and with the guys at school, and Queen’s and everyone else en route. It couldn’t happen doing it on your own. You need to have good team-mates and be in good teams, with people you like training with and playing with.”

The same applies today, and next week, and should lead to a recall for the South African series.

"It would be a tough tour to go on, after such a long season, but a fantastic tour too. But it won't happen if Ulster don't perform well. If Ulster lose the next two games and don't make the play-offs, and Leinster, Connacht and Munster do, and two of them make the final, and have an incredible game in the final, you'll see more of their players in the squad.

“So first and foremost, it has to be about what we do here. It won’t happen if you don’t enjoy what you’re doing, and do it well, with Ulster, to then make it happen with Ireland.”

Also, for the squad, management, redeveloped Kingspan Stadium and support levels Ulster have, to not make the play-offs would hurt badly. “Everyone in the squad knows that.” He cites the back-to-back defeats in February to Cardiff away and the Scarlets at home.

“They were two games that we should have nailed down, that’s six more points that would have had us nearer the top of the table. That’s why we’re under a bit more pressure now but we should be able to do well under pressure. This is where we like to be.”