Following on from his fine form in the 2007/08 season, he began the 2008/09 season in equally good form for Ulster; playing most of his rugby once again as an Inside Centre. His club form was so good that, even with a fit-again Gordon D'Arcy, he was named as Ireland's starting number 12 for the 2009 Six Nations. Although he lost his starting berth for the final two matches of the campaign, he was a valuable member of the victorious Ireland team that won the 2009 Six Nations Championship and Grand Slam, their first Grand Slam in 61 years. There was massive relief for Wallace after Stephen Jones missed a late penalty awarded when Wallace clumsily strayed offside at a ruck. His shirt that he wore (12) was made up of tiny signatures of Irish fans the first being his son.