Hartnett made his championship debut when he came on as a substitute in an ill-tempered Munster quarter-final defeat of Clare in 2007. Due to the suspensions of Donal Óg Cusack, Diarmuid O'Sullivan and Seán Óg Ó hAilpín following that game, Hartnett was handed his first championship start for the subsequent Munster semi-final. Cork lost to Waterford on that occasion but Hartnett became a regular member of the team before leaving the panel after the 2008 championship.
