He represented his native country at the 1984 Summer Olympics in Los Angeles, California, where he finished in ninth place in the men's marathon.
[6] Kiernan represented Ireland 17 times between 1975 and 1993, competing seven times in world cross-country championships, and won five Irish titles, from the 1,500m to the marathon, and also national inter-club and inter-county cross-country titles.
[citation needed] His running club in Dublin was Clonliffe Harriers.
Kiernan coached many athletes including Ciara Mageean and trained regularly in the UCD campus.
[8][9] Kiernan taught at St. Brigid's Boys School, Foxrock,[2] for approximately 40 years until retiring in June 2013.