Killeen has represented the Republic of Ireland at under-17, under-19, university and senior level. In 2010, together with Megan Campbell, Dora Gorman, Denise O'Sullivan, Ciara Grant and Clare Shine, she was a member of the Republic of Ireland U-17 squad that were runners-up in the 2010 UEFA Women's Under-17 Championship and quarter-finalists in the 2010 FIFA U-17 Women's World Cup. Killeen made her senior debut against New Zealand at the 2014 Cyprus Cup. She has subsequently represented the Republic of Ireland in their 2015 FIFA Women's World Cup and UEFA Women's Euro 2017 qualifying campaigns. Killeen also represented Ireland at the 2013 and 2015 Summer Universiades.