Career
He competed at the 1964 and 1968 Olympics in the mixed three-day event, individually and with the Irish team, with the best result of fourth place in the team competition in 1964. In retirement he helped design cross-country eventing courses, in particularly the course at Punchestown, Ireland, which was used for the 1991 and 2003 European Championships. He also headed the Irish junior (1981–1985) and senior (2007) teams.
In 1985 Brennan was awarded the FEI Gold Badge of Honour, and in 1997 inducted into the Irish Sports Council Hall of Fame
Brennan was born in Kilkenny, Ireland, and raised on the family farm in Dunnamaggin, where he trained in horse riding and animal husbandry.
He then moved to Dublin to improve his riding skills and study agriculture.