With injuries mounting, Donal Lenihan on three players Ireland cannot afford to lose.
Four years ago, Tadhg Furlong, with just one cap to his name, went to the World Cup as the fifth-choice prop.
Tucked in behind Mike Ross and Nathan White in the pecking order at tight-head, he was also designated as the emergency cover at loose-head for Cian Healy and Jack McGrath.
Thirty-three caps and three Lions Tests later, Furlong is now the cornerstone of the Irish pack, and on top form, the best tight-head in the game.
What makes him so valuable to the Irish set-up, apart from his outstanding scrummage ability, is the skillset he brings as an unlikely pivot in broken play.
As a tight-head, he carries the responsibility for making sure that not only are Ireland rock-solid in key defensive scrums, but also for creating an angle on the right-hand side off attacking scrums to buy vital space for his No 8 and scrum-half to attack.
Not only is he outstanding in all those key aspects of play, but he also manages to exert massive pressure on the opposition loose-head prop on their put-in.
What separates him from all others in this demanding position is the quality of his hands.
No other international prop could pull off the intricate swivel pass he provided Bundee Aki, with Johnny Sexton drawing the English defence expecting his trademark loop to be executed, only for Furlong to play Aki through a minute hole in the build-up to that classic try from CJ Stander in Twickenham in 2018.
Furlong also has presence in the tackle, is an explosive ball-carrier and highly-effective at cleaning out opposition bodies at the breakdown.
He offers Ireland so many advantages in key areas that Schmidt just cannot afford to lose him for any of the big games.
Sexton has always been a key performer for Ireland, central to everything that happens on the field. When he fails to fire on all cylinders, Ireland suffer.
Unfortunately, his injury profile hasn’t been great in recent years and losing him for the quarter-final against Argentina in 2015 was a massive blow.
After that defeat, Joe Schmidt swore he would never be exposed by losing Sexton again and set about putting his back up plan in motion.
However, his original choices in Ian Madigan and Paddy Jackson fell by the wayside for differing reasons meaning that the next in line, Joey Carbery, would have to be fast-tracked.
A move to Munster facilitated that and such was Carbery’s form in the opening 45 minutes against Italy last month, Schmidt was purring.
Then, disaster struck with Carbery’s ill-timed ankle injury. As his recently confirmed third choice, Jack Carty and next in line Ross Byrne are both shy on international exposure, Sexton has become more important than ever.
He has to be right for the key opening games against Scotland and Japan and his 64 minute appearance against Wales could not have been better timed.
Sexton is the eyes and ears of the coach on the field and his leadership, authority, and direction was badly missed against England in Twickenham.
He is also a key defender, a major influencer from a communication perspective in that facet, always directing those around him.
The soft tries that England scored off set pieces last month, when a number of players were guilty of making poor defensive reads under pressure, would not have happened had Sexton been in situ.
The uncertainty surrounding Carbery’s wellbeing for the opening games means that Sexton has now become even more crucial to Ireland’s cause than he was four years ago.
A comparative rookie alongside Sexton and Furlong, Ryan’s progress has been spectacular since winning his first cap, a try-scoring debut off the bench against USA in 2017, before featuring for Leinster.
This young lock always appeared destined for greatness and has wasted no time in getting there.
Much debate has centred on the composition of Ireland’s second-row but it is a measure of just how far Ryan has travelled that the discussion been whittled down to who should partner the towering Leinster man.
Ryan has been a model of consistency since he established himself in the side with his tackle count, ruck cleanouts and carries, frequently topping the charts.
Any fear of a fall-off in impact due to second-season syndrome was quickly discounted with Ryan emerging from a disappointing Six Nations championship as Ireland’s player of the season.
His ability to operate comfortably on both sides of the scrum, along with his line-out flexibility and explosiveness is another major asset as it enables Schmidt to alter his selections to suit the opposition.
Despite starting only his third season of international rugby, Ryan has already become a source of inspiration in the Irish dressing room.
After the depressing defeat in Twickenham, the fact that Ryan was as influential as ever in his first two outings of the season against Wales has proved a massive boost.
Schmidt’s contentious call to omit Devin Toner has served to heap even more pressure on Ryan with the responsibility for calling the lineout now also resting with him.
A future international captain, Ryan has real presence and if Ireland are to make a serious impact at this World Cup, then it is imperative that the 23-year-old remains fit for the duration of the campaign.
This article appears in Friday's 40-page Irish Examiner Rugby World Cup preview magazine.