Career
Harkin has played international football for Northern Ireland at both youth and under-20 level Started his playing career at Wycombe Wanderers when he was the first player to progress from the youth team in the summer of 1996. His first game was against Barnet in the second round of the 1996-1997 Field Artillery Cup where he came on as a substitute in the 60th minute.
His first goal did not arrive until 26 August 1997 with a 25-yard chip past the Fulham goalkeeper, a result that ended in a 4–4 draw.
Harkin was also on the score sheet in the 5–2 against Iceland. Local media stated that Aston Villa offered Wycombe £800,000 for Harkin, but the deal ended when he picked up an injury.
At the end of the 2000-2001 season, at the age of 21, the then manager, Lawrie Sanchez, decided not to renew his contract with the club stating that there was no future for him at Wycombe and maybe a fresh start at another club could restart his football career. Harkin signed with Carlisle on 20 August 2001 on a month deal but was not renewed.
Harkin moved south and joined the Hampshire team of Aldershot Town, who were in the Isthmian League at the time.
He only made 2 appearances before going north to Nuneaton Borough on 2 January 2002, before returning south to the Sussex team of Crawley Town in June 2002. In 2005 he had a short stint at Forest Green Rovers before going to Lewes. Mo joined his second Hampshire team, Havant & Waterlooville, in June 2006 who were playing in the Conference South.
He featured in the fourth round tie against Liverpool at Anfield.
Although Havant lost 5–2 to the Premier League outfit, he was the man on the match. He joined the newly promoted Conference National side Eastbourne Borough in July 2008. on a free transfer.
He made 6 starts for Eastbourne, scoring 1 goal. On Boxing Day, he suffered with a mystery vision problem whilst warming up against Lewes.
Harkin was released by Eastbourne at the end of January 2009 transferring to Southern League Premier Division team Farnborough.
During his time at Kingstonian he was loaned out to Hanwell Town.
