[3][4][5][6] Between 1997 and 2000 Jermyn was a member of the Ashton School team that played in four successive All Ireland Schoolboys Hockey Championship finals.
[1] He was also a member of the Cork Church of Ireland team that played in the 2014–15 final, losing 2–1 after extra time to Banbridge.
[9] In 2015–16 Jermyn scored eleven goals, including three hat-tricks, for Cork Church of Ireland in the Men's Irish Hockey League.
Together with Iain Lewers, Eugene Magee and Barry Middleton, he was a member of the HGC team that finished as runners-up in the 2007–08 Euro Hockey League.
The club agreed fly him to Belgium on weekends for matches only, allowing him to continue his career as a solicitor in Cork.
[23] Jermyn was also a member of the Ireland team that won a 2012–13 Men's FIH Hockey World League Round 1 tournament;[18] on 7 September 2012, in an opening pool game against Wales, scored a goal that tied him with Stephen Butler as Ireland's all-time leading goal scorer, breaking the record the next night when he scored his 80th all-time in a game against Austria.
In March 2018 he was appointed assistant diocesan registrar by the Bishop of Cork, Cloyne and Ross, Paul Colton.