O'Riordan made his senior championship debut for Cork on 9 June 1946 in a 2-9 to 2-1 Munster quarter-final defeat of Clare. He immediately became a regular member of the starting fifteen and later won his first Munster medal following a 3-8 to 1-3 defeat of Limerick. On 1 September 1946 Cork faced old rivals Kilkenny in the All-Ireland decider. While some had written off Cork's chances, they took an interval lead of four points. With ten minutes remaining Cork's lead was reduced to just two points, however, goals by O'Riordan and Joe Kelly secured a 7-6 to 3-8 victory. It was O'Riordan's first All-Ireland medal.