McCarthy first played hurling for the Feenagh junior team as an 11-year-old before gaining further experience as a boarder at Rockwell College. Here he lined out in back-to-back Harty Cup finals in 1934 and 1935 before winning an All-Ireland medal as part of a star-studded Munster Colleges team that also featured Jack Lynch, John Keane and Connie Buckley. As a medical student at University College Dublin, McCarthy was involved in several Fitzgibbon Cup campaigns and claimed a winners' medal in 1941. After lining out at midfield in the early part of his career he switched to being a goalkeeper on the Limerick junior team that won the 1935 All-Ireland Junior Championship. McCarthy's performances at junior level earned at immediate call-up to the Limerick senior team with whom he won National League, Munster and All-Ireland honours in his debut season in 1936. He won a further two National League medals over the course of the following two seasons before winning a Limerick Junior Championship with Feenagh in 1938. McCarthy won a second set of Munster and All-Ireland medals as a full member of the starting fifteen in 1940. He also earned inclusion on the Munster inter-provincial team and won back-to-back Railway Cup medals in 1942 and 1943.[citation needed]
