Rory Best said Ireland didn't have the "right answers" to the questions posed by Japan as the Rugby World Cup hosts pulled off a monumental shock in Shizuoka.
"We knew how tough it was going to be," said the Irish captain after the 19-12 loss.
"Anyone who is utterly shocked hasn't seen how good they are. We came in with a game-plan and we felt we were really prepared but give credit to that team, they played really well.
"We made a few mistakes, a few unforced errors, we were on the wrong side of the penalty count.
"Japan played really well. They posed a lot of questions but unfortunately we couldn’t come up with the right answers."
Ireland moved into a 12-3 lead by the 22nd minute but failed to score again and repeatedly kicked possession back to the free-running Japanese.
"When you play the host team it’s always tough," said Best.
"When you play a team like Japan... we didn’t start exactly how we wanted but we got the right side of the scoreboard and then we let them play and when you let a team like that play you can sometimes be chasing shadows
"They attacked us minute after minute, we are not happy with how we played.
"We’ll have to dust ourselves down and get better."