 By November 2006, still just 17 years old, Cathcart had been promoted to the Northern Ireland Under-21 team, and was named in the squad for a friendly against Germany on 14 November 2006.In February 2007, Cathcart was selected in the European squad for the 2007 Meridian Cup, a competition between two "all-star" Under-18 teams from Europe and Africa.Since his first call-up, Cathcart has been a regular name in Northern Ireland Under-21 squads, and has so far made seven appearances for the team, the most recent being a 3–0 defeat by Germany on 5 September 2008.