Duffy is a former international rugby unionfootballer who primarily played at full back. He last played professionally for Irish provincial side Connacht Rugby, in the Pro12, and represented Ireland at full international level.

In November 2001, he won his first Ireland A cap against New Zealand at Ravenhill and his performances for Harlequins in the 2003–04 European Challenge Cup saw him called into the senior Ireland squad for the first time for the 2004 tour to South Africa. He won his first cap on that trip when coming on as a replacement during the second test. In February 2005, he won his second cap when again coming on as a replacement against Scotland, marking the occasion by scoring his first international try. He was selected to play at outside centre for Ireland in both summer Tests in Japan in 2005, scoring two tries in the second Test.