Larkin was still a member of the under-21 team when he was added to the Kilkenny senior panel in 2005. Kilkenny were back in form that year, with Larkin winning a first National Hurling League medal following a huge 3–20 to 0–15 victory over Clare. On 12 June 2005 Larkin made his championship debut in a 6–28 to 0–15 Leinster semi-final trouncing of Offaly. "The Cats" later struggled against a wasteful Wexford side, however, a 0–22 to 1–16 victory gave Larkin a first Leinster medal. While a third successive All-Ireland showdown with Cork seemed likely, Galway defeated Kilkenny in the All-Ireland semi-final in one of the games of the decade.