Head coach Ian Foster said injuries demanded the change, but he was confident Barrett could handle the role in a position New Zealand has struggled to fill since Jerome Kaino's retirement.
Barrett said he was happy to be playing on the blindside flank. He has played there a few times already this season, and in the past, so it wasn't too unfamiliar for him.
He said the lineout role required of him was similar. He would have an extra gear after scrums due to not being in the engine room, while his around the field game wouldn't change.
It is his first start for the All Blacks in the position since the All Blacks' semifinal loss to England at the 2019 Rugby World Cup Barrett said there were lessons from that experience for all involved.
They hadn't won the first 20 minutes of that game and didn't win the physicality battle either.
"England brought it then and we're expecting Ireland is going to be right into it from the start so I am excited by that," he said.
Coaches Ian Foster and John Plumtree had talked about the possibility of him playing in the position at their training camp in the Bay of Islands last week. His response was if he was in the playing XV it didn't matter in what capacity as he would be excited to play.
He took confidence from his play at the end of the Crusaders' Super Rugby season. He wanted and to build on that for the All Blacks.
Barrett said Ireland took a lot of momentum and confidence from their lineout, and he felt if the All Blacks could put them under pressure in that area it could stop a lot of their game.
Hooker Codie Taylor said the week's preparation had been different. No one expected to lose their coaches but similar things had happened during the Super Rugby season, and the players had rolled with the punches.
Contingency plans were worked on, and the training had been smooth, he said.
Last year's loss to Ireland in Dublin was still on the minds of those involved. They hadn't responded to Ireland's physical game, especially in the second half of the contest.
"We expect a lot of the same, they really back their style of play. They're a great attacking team and they put the ball on a plate for the likes of Jonny Sexton. It's up to us as forwards to make sure we slow the ball down as best we can and do a job in that area because when they get a roll on it is quite hard to stop," he said.
Former Ireland coach Joe Schmidt had not been passing information on from his time and had been more involved in helping the All Blacks get their game in the right place individually and collectively.
Taylor said they had been working on a shift in their mindset at the contact zone and technical matters.
"Being in the game and not on the ground is going to be crucial for us," he said.