Kirwan played at wing for 96 appearances (including 63 test matches) as an All Black from 1984 until 1994. Kirwan played a major role in the All Blacks’ 23 test unbeaten run from 1987–1990, scoring 10 tries in five tests against Wales and Australia during 1988. In all he scored 35 test tries for the All Blacks, and 67 tries in total for appearances with the All Blacks, the latter which continues to be the New Zealand record. Kirwan's record of 199 first class tries continues to be the New Zealand record for first class rugby.
In 2002, Kirwan moved to Italy to become the coach of the Italian national rugby team, which under his guidance recorded historic victories over Wales in 2003 and Scotland in 2004. After a winless 2005Six Nations campaign, he was relieved of his managerial duties on 8 April 2005.