Rúaidhrí O'Connor: Ryan versus Itoje could become era-defining rivalry as they prepare for round two at the Aviva
The parallels in their career trajectories are clear, even if they are different players operating in the same department.
What is obvious is that Maro Itoje and James Ryan are cut from the same cloth and, injury-permitting, they will be the fulcrum of the England, Ireland, Saracens and Leinster packs for the next decade.
Both marked out as future international captains from a young age, they were prodigious underage talents fast-tracked into international rugby - a realm in which they immediately thrived.
Now 23, Itoje - two years Ryan's senior - first burst on to the scene to great acclaim and with an insatiable desire for silverware.
In his first two full seasons as a senior player, he won two Six Nations - the first a Grand Slam - two Champions Cups and a Premiership title, collecting the European Player of the Year award in 2016. He was part of a clean sweep tour of Australia with England and played in three Tests of last year's drawn Lions series in New Zealand.
For the first 16 months of his senior career, he didn't lose a game - coming out on the right side of the result 31 times in succession until finally tasting defeat in September last year.
Ryan has some catching up to do, but his rise mirrors Itoje in more ways than one. He was a sensational U-20s player, captaining Ireland to their win over New Zealand and the 2016 World Cup final in England, before being fast-tracked into the international set-up and winning the Grand Slam at his first attempt two weeks ago.
A hamstring injury ruined what should have been his breakthrough season last year, but since making his first senior appearance on Ireland's summer tour.
That win over the US Eagles was the first of 18 senior victories he has clocked up to date for club and country. His injury profile has curtailed his impact with Leinster for whom he has started just five times, coming off the bench for a further five involvements.
He has eight Ireland caps, five of which came from the start and while he has had to be patient with Devin Toner and Scott Fardy linking up well in the provincial ranks he returns to Leo Cullen and Stuart Lancaster a proven international performer.
Lancaster brought a young Itoje into his England squad before the 2015 World Cup but didn't select him, while he is working with Ryan at Leinster. Not that he's seen too much of the 21-year-old.
"I haven't actually coached him loads and loads of times," he conceded.
"He's not played in many of the games since I've been here, certainly he didn't play last year.
"They're both high-quality players although they play slightly differently. They've got the same drive, the same mentality of wanting to succeed.
"James suffered last year from that injury but it was a blessing really in that allowed him to develop the physical size and physicality you need at international level.
"That, plus his confidence and belief has grown through the Champions Cup for us which had led into Ireland games.
"They're both similar in terms of profile and determination to succeed. Think they'll be going head-to-head for a few years yet.
"Playing big games, you can't accelerate that, you've got to go through it.
"There is no doubt James will be a better player for going through the Six Nations and everything that has happened for him.
"That said, I've been involved England, and given lots of lads their first cap and they go on this rise but then they get more scrutiny, people give you more attention and start to look at your game in more detail.
"That is when the challenge comes and that is the same for James or Maro or any of them."
Itoje has reached that stage now, with many pundits in England wondering whether the first two seasons have taken their toll on his young body.
His exuberance on the pitch, which manifests itself in the celebration of penalties, can rub some up the wrong way and looked plain silly at Twickenham a fortnight ago when he was guilty of plenty of poor discipline of his own as he handed Ireland the initiative.
With Clive Woodward questioning his fatigue levels, Itoje returned to Saracens colours last weekend and put in a man of the match shift, celebrating his try by pretending to take a nap on the pitch in a pointed response to those suggesting he needs a break.
Ryan, meanwhile, was given the weekend off and is back in blue this week with more expectation on his shoulders than he's had before but no guarantee he will be in the starting XV.
If handed the responsibility he'll take it on his broad shoulders.
Nicknamed 'The Big Cheese' by his team-mates, he is a serious young man with a steely determination and a keen desire to succeed.
There are concerns about his durability, but he refutes the idea that he is injury-prone and will be turning up for work this week hungry to follow in Itoje's foot-steps up to the European podium in the weeks to come.
He won the first battle between the two of them at Twickenham, Saturday's Champions Cup quarter-final is round two.
The way this duo are going, there will be many more to come.
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