Andy Farrell’s men quelled Japan’s challenge with two final-quarter penalties from Joey Carbery, who marked his first Test appearance since the 2019 Rugby World Cup with 14 points.
The lead changed hands on seven occasions in a real nip-and-tuck tie, Ireland edging the try count five to four as Chris Farrell, Stuart McCloskey, Finlay Bealham, Josh van der Flier and Jacob Stockdale all crossed.