He, and horse Going Global, are due into the individual showjumping arena at around 4 o'clock Irish time.
Boxer Michael Conlon also gets his Olympic Games underway this afternoon.
The Belfast Bantam faces Aram Avagyan of Armenia in round 16 at around 3:30pm.
Padraig Harraington is 7 shots off the pace ahead of the final round of the Men's Golf tournament.
He is 5-under par with leader Justin Rose 12 under par.
Seamas Power resumes from 1 under par.
It's women's marathon day too.
Fionnuala McCormack, Lizzie Lee and Breege Connolly will all be in action from 1pm.
