O'Neil took up disability sport whilst at a sports camp in New York City at the age of twelve. She first represented Ireland at a major international tournament at Birmingham during the 1998 IPC Athletics World Championships, winning gold in the F51 women's discus throw. O'Neil competed at three Summer Paralympic Games during her career, starting at the 2000 Summer Paralympics at Sydney. Initially specializing in the discus, show progressed in her later career to also entering club throw events. Although not competing in Athens, O'Neill was in the Ireland team for both the 2008 Games in Beijing and 2012 Games in London. Her only Paralympic podium finish came at London, where she won a silver in the &lt;a href="Athletics%20at%20the%202012%20Summer%20Paralympics%20%E2%80%93%20Women%27s%20discus%20throw"&gt;discus throw - F51/52/53&lt;/a&gt;.