Ó Cróinín made his debut on the inter-county scene at the age of sixteen when he was picked on the Cork minor team. He enjoyed two championship seasons with the minor team, before later joining the under-21 side, however, he ended his underage career with championship success. Ó Cróinín made his senior debut during the 1998-99 league. Over the course of the next seven seasons, Ó Cróinín established himself as a key player for Cork and won two Munster medals and one National Football League medal. He played his last game for Cork in July 2005.