Byrnes was recruited from the Murray Bushrangers and was the last draftee selected by the Geelong Football Club in the AFL rookie draft in 2002.

As a second year rookie, he was promoted to Geelong's senior list for the remainder of the 2004 season, replacing James Kelly who was out for the rest of the year after breaking his leg against the West Coast Eagles.

Byrnes was used effectively off the bench mainly in his first 2 seasons on the senior list, and earnt a rising star nomination in 2005 with a 24 possession and 2 goal game against the West Coast Eagles at Subiaco.

The speedy-left footer had an inconsistent season in 2006, with fluctuating form, and the inability to break into a strong team seeing him being put in and out of the playing squad.

He cemented a spot in the team, missing only two games due to injury, and was picked to play in the 2009 AFL Grand Final, where he had 16 possessions, kicked one goal and became a two-time premiership player.