Ireland were ahead after six minutes when Paul Ryan put in a fine tackle and turnover in the Japan 22, before offloading to wing Paddy Brophy, who easily finished off in the corner.
Minutes later Brophy scored again when he was on the end of a fine back move and after impressive handling by Jamie Smith he was able to outpace the Japanese covering defence.
Ireland began to throw the ball about with self assured confidence and it was Eoin O’Malley, starting at fly half after playing his previous games at centre, who finished off a great piece of work by hooker Michael Sherry.
Japan kept attacking and after captain and scrum half Tappei Higashide took a quick tap penalty, his side were awarded an attacking lineout from which they mauled Jumpei Ohira over the line.
Ireland scored their fourth try after good work from the Irish pack as half time approached. Smith ran a good line, before passing to winger Alan Gaughan whose offload allowed Patrick Mallon to score from close range.
Japan began the second period brightly and it was only due to excellent scramble defence by Niall Morris that Japan did not score.
Their pressure eventually paid off soon after as Japan again mauled over the Irish line following a well taken lineout with Norimasa Kuribayashi touching down.
O’Malley sealed the win for Ireland when he went over for his second try of the match, which he converted himself.
Ireland will now face Scotland for ninth place, the second time they have played in this tournament, whilst Japan will play Fiji to avoid finishing bottom.
