Following his good performances for the Llanelli Scarlets in both the Celtic League, and the Heineken Cup during the 2005–06 season, he was called up to the national squad for the November Test series. He made his début as a replacement against the All Blacks on 5 November at the Millennium Stadium. Wales lost 3–41. He also played in the win over Fiji, and the defeat by South Africa. He has since played during the 2006 Six Nations Championship, and earned a further two caps in a mid-year series against Argentina. His sole appearance in the 2006 Autumn internationals was on the wing against the Pacific Islands. Byrne was a revelation in the 2008 Six Nations Championship tournament at full back and a key component of Wales' Grand Slam.