Ienaga began his career as a trainee with Gamba Osaka, one of the leading clubs in Japan's J. League. He broke into the senior squad in 2004 and helped the club to their first J-League title the following year. Ienaga joined J-League rivals Oita Trinita on a season-long loan in 2007. In December 2008, Ienaga joined English club, Plymouth Argyle, on trial. Plymouth attempted to sign the player in January 2009 but he was denied a work permit so they were forced to withdraw from talks. In December 2010, he signed with Spanish Primera Division club RCD Mallorca in a five-year deal. And scored his first goal for the club in home game against Sevilla on 9 April 2011.