Brian O'Driscoll rounded off a flowing backline move seven minutes after halftime for a converted try that put Ireland 19-6 ahead and Keith Earls touched down in the corner twice as Italy's hopes of a first quarterfinal evaporated.
Ronan O'Gara experienced none of the kicking problems that afflicted players in the other three pool matches under the roof of Dunedin's Otago Stadium, succeeding with six of his seven place kicks at goal and hitting the post with the other.
Ireland finished atop Pool C and will play Wales in Wellington on Saturday.