But IRB judicial officer Judge Wyn Williams, who deferred his decision overnight, has opted against taking additional action as Nonu did not drive D'Arcy into the ground.
Players found guilty of a dangerous tackle can be banned for between three and six months, with the IRB anxious to stamp out such challenges.
Referee Jonathan Kaplan reacted to the second-half incident, which brought back memories of the double spear-tackle by Tana Umaga and Keven Mealamu on Brian O'Driscoll during the first British & Irish Lions Test in June, by awarding Ireland a penalty and Williams ruled this was sufficient punishment.
An IRB statement read: "His Honour Judge Wyn Williams QC (Wales) concluded that the evidence did not demonstrate that the player drove his opponent into the ground after lifting in the tackle.
"Furthermore it was also accepted that although the player had committed a dangerous tackle on Gordon D'Arcy, the referee had seen the incident and that his reasons why he dealt with the tackle by awarding a penalty and a caution were not wrong.
"Therefore no penalty was imposed and the player is free to continue playing."
Williams came to his conclusion after watching a video recording of the tackle, examining Kaplan's report, listening to Nonu's account and accepting representations from the player's legal team and New Zealand coach Graham Henry.
The challenge was condemned by Henry and Ireland boss Eddie O'Sullivan after the match but both agreed the tackle was clumsy rather than malicious, although O'Sullivan demanded that any instances of spear-tackling should be
Nonu, who made his first appearance of the tour in the 45-7 drubbing of Ireland, has been overlooked by Henry for Saturday's clash with England at Twickenham but will come into contention for Scotland seven days later.
Umaga and Aaron Mauger are Henry's first choice midfield partnership but Nonu looks set to link up with Conrad Smith at Murrayfield where New Zealand are expected to complete a successful Grand Slam tour.