First capped at Schools level, Reddan represented Ireland at U19 and Student levels, making his senior international debut in the 2006 Six Nations as a replacement against France at the Stade de France. His first start came against Argentina during the summer tour that year. He displaced Peter Stringer during the 2007 Rugby World Cup in the Irish team after Irelands disastrous start to the campaign, playing in their two final group games, losses against Argentina and France. He made his first Six Nations start for Ireland in their 16–11 victory over Italy at Croke Park in 2008 earning man of the match for his performance. He was then replaced by Tomás O'Leary as first choice Ireland scrum half. He was named in the Irish squad for the 2009 summer tour to America.