The 23-year-old, who is equally comfortable at blindside flanker and looked set for a role as an impact replacement six weeks ago, has not only forced his way into the side but quickly become one of Joe Schmidt's indispensable men.
The big Ulsterman demonstrated his aggression in attack with a try in Ireland's opening victory over Canada - just his sixth international start - and his physicality in defence in a man-of-the-match display against Italy on Sunday.
Despite performing so well at the World Cup and standing out even in an Ireland squad of real depth, Henderson has retained his laid-back nature as well as a fondness for grabbing 40 winks at any given opportunity.
"Few things will keep me awake at night, it's harder to keep me awake during the day," Henderson told reporters last week.
"I like a wee nap. Paul (O'Connell) has been commenting on the amount of naps I get in in the afternoon.
"With Joe and all his meetings, I have to have alarms constantly set."
Recent outstanding performances - particularly in the World Cup warm-up games against Wales - catapulted Henderson above the other relaxed man of the squad, Devon Toner, who was an ever-present during Ireland's back-to-back Six Nations triumphs.
Henderson believes Schmidt had no real reason to give him a run in the starting side because Toner "always plays well and rarely doesn't have a good game".
The level-headed "beast" clearly has modest streak, however, as his game has virtually everything, even after just three years of professional rugby.
Comparisons to another Northern Irishman, the British and Irish Lions most capped player Willie John McBride, are well founded.
Henderson's effectiveness at the ruck is up there with the best back row operators in the tournament, something Henderson puts down to his growing game awareness or, as he puts it, knowing "when to have a wee dip".
Schmidt, who used Henderson off the bench throughout the last two Six Nations campaigns, has seen improvements in the 1.98 metre-tall lock's lineout and his "first rate" ability to add value at the scrum, complementing his physical attributes.
"I think with Iain, he's always been a super athlete but I think his understanding of the game has improved," Schmidt said after the Italian win that sealed Ireland's quarter-final place.
"He's starting to match his athleticism with his understanding and then starting to really deliver in more of the elements of the game.
"He adds real value for us, he's pretty keen to continue adding value and we're pretty keen that he does, to be honest."
(Editing by Nick Mulvenney)
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