Merrin grew up playing rugby league with the Shellharbour Sharks like fellow front-row forward at the Dragons, Dan Hunt. Merrin claims another Shellharbour legend, Australian representative Luke Bailey, was "one of [his] idols growing up". In 2008, Merrin played for the Dragons Under 20s team in the Toyota Cup, scoring 2 tries for the season. He was selected as the Dragons' Toyota Cup Player of the Year in 2008, and in 2009 was selected to play First Grade for the Dragons, being chosen on the interchange bench. He didn't manage a single minute in the match which the Dragons won 25-12.
He was a prominent squad member of the 2010 premiership-winning Dragons team, and came from the bench to play in 2010 NRL Grand Final's final minutes.