That New Zealand eventually pulled clear was down to the clear superiority of their backline.
Tevita Li scored New Zealand's first try, his sixth of the tournament. He added another and could even have scored four.
Man of the match Richie Mo'unga, who started at first five and finished at fullback, scored two tries.
Lock James Tucker had an equally eventful night, sinbinned once for pulling down a maul, crossing the line twice for one try, and winning his share of lineouts. Flanker Lachlan Boshier was also in prime form.
The next two New Zealand tries originated from Damian McKenzie high ball takes, the Irish persisting with the Garryowen, as South Africa did in the semifinal.
Ireland's pack was up to the task, their solid scrum taking no nonsense in the forward exchanges.
Australia clinched fifth position, two better than they managed in 2013, coming from behind to beat France 34-27 in the early Eden Park match.
New Zealand 45 (Tevita Li 2, Richie Mo'unga 2, James Tucker, Mitch Drummond, Lachlan Boshier tries; Damian McKenzie 5 cons)
Ireland 23 (Penalty try, Oisin Heffernan; Ross Byrne 2 cons, 3 pens) HT: 21-23