Often referred to as "Big" Seanie to differentiate him from a smaller teammate of the same name, McCarthy was 5 foot 11 inches tall but in his early days especially was known for his slim build.
McCarthy scored on his debut in a 1–1 draw with Shamrock Rovers and helped Cork to the title in his first season (1940–41) as well as winning the FAI Cup after a replay with Waterford F.C.. McCarthy scored eleven league goals that season with a further three in the Cup, including a goal in the replayed final.
McCarthy's success continued and he helped Cork United to four league titles in five seasons.
Financial problems saw Cork United fold in 1948 which resulted in McCarthy moving to Dagenham to work in the Ford factory there.
Success at Dartford saw McCarthy sign for Bob Wright at Bristol City in the summer of 1949 though he time there was shortlived and he returned to Cork soon afterwards.