'Below the line': The cheap shot that sparked All Blacks captain Scott Barrett

All Blacks captain Scott Barrett has revealed what exactly set him off in the first half against Ireland as tempers flared early in the contest.

Barrett was seen scragging opposition lock Joe McCarthy by the collar and locked into a war of words as players from both sides tried to separate them.

The All Black captain told media after the match that he had spotted a "below the line" incident that forced him to step in and deal with.

"I don't usually take exception but I saw something that I guess was below the line for me," Barrett said.

"I had to make a point that, it's not tonight you are targetting our No 10 [McKenzie] like that.

"They play hard, they're a great team, physical team, great with ball-in-hand, it was just the one moment there."

Barrett rushed into the forming melee and appeared to mouth "don't you do that" at McCarthy multiple times.

The enforcer clarified that he saw what he believed to be a high shot with a shoulder to a defenceless player on the ground, chief playmaker Damian McKenzie.

"I was getting up from a ruck, from where I saw it it looked like Damian was on the ground and Joe [McCarthy] cleaned him up," he said.

"From where I saw it, it looked like it was around his head. Whether it was or not, I'm not sure.

"I took exception to that in that moment, it may not have been but I'm not too sure."

"It looked like a shoulder to a man on the ground who was our 10."

The actions sparked the All Blacks in a positive way, Barrett was one of the best on ground for the All Blacks pack and Damian McKenzie ended up being named man-of-the-match for piloting the victory.

McKenzie kicked six penalties and had a hand in the lead up to the try to Will Jordan as the All Blacks beat the number one ranked side 23-13 in their own backyard.