Cillin Greene represented Ireland at the Tokyo 2020 Olympics as part of the mixed 4×400 meters relay team. The team set a new national record in the heats, advancing to the final where they finished eighth.

In June 2021, Greene clinched his first national 400 meters title at the Irish Senior Championships with a time of 46.38 seconds. Earlier that month, he recorded a personal best of 46.18 seconds at a meet in Geneva, placing him 10th on Ireland’s all-time list for the event.