Aki was ordered off by referee Nic Berry after 29 minutes for a tackle on Ulupano Seuteni at Fukuoka Hakatanomori Stadium, where Ireland needed a bonus-point win to be assured of a place in the last eight on Saturday.
Ireland were already in command at that stage following tries from captain Rory Best, Tadhg Furlong and Johnny Sexton.
Sexton's second try secured the bonus point just before half-time and Ireland dominated the second period, with Jordan Larmour, CJ Stander and Andrew Conway also going over.
