He progressed through the various age grades, winning a Cork Colleges U16HC medal in 2006, before later playing on the school's Harty Cup team.
[5] He later progressed to UCC's Fitzgibbon Cup team and claimed a winners' medal in that competition after a one-point extra-time defeat of CIT in the 2012 final.
Harnedy captained Imokilly to their first Cork SHC title in 19 years in October 2017, after scoring 1–01 in the 3–13 to 0–18 defeat of Blackrock in the final.
[10] Harnedy retained the captaincy the following year and won a second consecutive Cork SHC title after top-scoring with 1-06 from play in the 4–19 to 1–18 defeat of Midleton in the final.
In October 2021, Harnedy top-scored with 0-06 when he captained St Ita's to their first ever East Cork JAHC title after a 1-14 to 1-05 defeat of Erin's Own in the final.
[15][16] Harnedy failed to secure a place on the Cork panel for the subsequent 2011 National Hurling League campaign.
[24] He sustained a hamstring injury during Cork's 1–24 to 0–17 defeat by Waterford in the 2015 National League final and was subsequently sidelined for a period.
[31] In August 2021, Harnedy scored four points from play from centre-forward when Cork suffered a 3–32 to 1–22 defeat by Limerick in the 2021 All-Ireland final.