Hennessy first played hurling at juvenile and underage levels with the Kilruane MacDonaghs.[2] He is the only player to have lined out in six consecutive divisional under-21 finals and is one of only a handful of players to have won four consecutive Tipperary U21AHC titles with the club from 1973 to 1976. Hennessy was the youngest ever Kilruane player to line out in a North Tipperary SHC final when he did so as a 17-year-old in 1973. He came on as a substitute for Enda Hogan at midfield when Kilruane MacDonaghs won the All-Ireland Club Championship title in 1986[3][4]