Killeen studied Radiography at University College Dublin[2] While attending University College Dublin, Killeen represented the women's association football team at intervarsity level, playing in teams alongside Ciara Grant, Dora Gorman and Julie-Ann Russell.
[7] While attending UCD, Killeen and Ciara Grant also played for Raheny United in the Women's National League.
[11] Killeen also helped United win the 2015 WNL Cup and also represented the club in both the 2013–14 and the 2014–15 UEFA Women's Champions League campaigns.
[17] Just before the 2018 Women's National League season, Killeen decided to stop playing soccer, in favour of Gaelic football.
[12][1][19][20][21][22] As a schoolgirl Killeen played Ladies' Gaelic football with Scoil Uí Chonaill CLG and Raheny GAA.
[25] In 2011 Killeen was part of the Dublin team that reached the All-Ireland Under-18 Ladies' Football Championship final, losing to Cork.
[31] In June, she sat alongside Chief Medical Officer Tony Holohan and spoke about her experience at the daily Department of Health press briefing.