Ireland edged a thrilling back-and-forth match with Japan 39-31 before 3,000 entertained fans at Lansdowne Road on Sunday.
The lead changed five times alone in a breathless first half that Ireland finished 19-17 ahead. Japan then went back in front for a fourth time and threatened a repeat of its historic first win over the Irish at its home Rugby World Cup in 2019.
But Ireland gradually asserted control and managed possession better. While Japan closed to trail just 33-31, Joey Carbery's two penalties gave Ireland a safe buffer against the tiring Japanese with 12 minutes to go.
Japan rued a try disallowed and a penalty hitting the post.
Ireland was naturally sloppy without 15 regulars either with the Lions, injured, resting or retired. But a second-string team of impressive depth and strength gelled with time and made headway with powerful carries, great running lines, and halves who used their forwards' edge to play with veteran panache.