Killeen spent his first season out of college in France, playing for
Union Carquefou-Sainte Luce in 2009–10. In October 2010, Killeen had a one-game stint with the
Westports KL Dragons before joining the
Cheshire Jets in early December 2010. He lasted a month with the Jets, appearing in just four games for the club. Killeen joined his hometown team
UL Eagles for the 2011–12 season and helped them win the league championship and national cup. He returned to Eagles in
2012–13 and helped the team win back-to-back titles. In July 2013, Killeen signed with Beyssac Beaupuy Marmande for the 2013–14 season, returning to France for a second stint. He scored 329 points for the team. In July 2014, Killeen signed with
Templeogue for the
2014–15 season, returning to the Premier League for a second stint. He continued on with Templeogue in
2015–16, but missed the first half of the season due to a knee injury. On 30 January 2016, he helped Templeogue win their maiden National Cup trophy with a 78–75 win over
Swords Thunder in the final. In April 2019, Killeen was named Super League Player of the Year after leading Templeogue to the Champions Trophy title. Killeen parted ways with Templeogue following the 2021–22 season. Killeen is set to play for the
Limerick Sport Eagles in the National League Division 1 in 2022–23. ==National team career==