Slow-starting Ireland led 17-10 at the break after an underwhelming first half, with Jack Crowley and Nick Timoney crossing for the hosts.
Despite losing Jacob Stockdale to a yellow card immediately before half-time, Ireland wore Japan down after the break with Andrew Porter, Gus McCarthy, Paddy McCarthy and O’Brien scoring tries to add gloss to the scoreline.
Despite a more assertive second-half display, Ireland skipper Caelan Doris recognised his side were “a bit desperate” during the first 40 minutes.
“I felt we went inward a little bit in the second half,” said Doris, who made his first start in six months after recovering from his own shoulder injury.
“There were quite a few examples of us forcing things. That’s probably the challenge when there are quite a few changes.
“There’s an eagerness to stamp yourself on the game individually and as a result we were a bit desperate at times.”
