On 30 December 2018, it was announced that Harkin had returned to Derry City on a two-year contract, as part of a swap deal that saw Ben Doherty moving in the opposite direction.[10] On 14 September 2019, he featured in the 2019 League of Ireland Cup final as his side were beaten in a penalty shootout by Dundalk, despite scoring his penalty in the shootout.[11] On 29 March 2024, he made his first appearance in nearly two years in a 1–0 defeat at home to Galway United, having recovered from his two separate Anterior cruciate ligament injuries.[12][13] On 29 July 2024, Harkin returned to former club Coleraine on loan until January 2025.[14]
Sligo Rovers
On 29 July 2025, it was announced that Harkin had signed for fellow League of Ireland Premier Division club Sligo Rovers until the end of the season, with the option of an additional 12 month contract.[15]
International career
He was called up to the Northern Ireland U21 team in March 2018, remaining an unused substitute on 26 March 2018 in a 0–0 draw at home to Iceland U21.[16]
Personal life
Harkin is nicknamed "Jackie" since childhood, after martial artist Jackie Chan, as he was told he was a good fighter as a child.[17]