CAS says rider Denis Lynch has challenged Horse Sport Ireland's nomination of Cian O'Connor in a two-rider team in individual jumping. The competition begins Aug. 4 at Greenwich Park.
Lynch was originally selected then withdrawn after his horse, Lantinus, was disqualified from a recent event on health grounds.
O'Connor won gold in show jumping at the 2004 Athens Olympics but was stripped of his medal after his horse tested positive for a banned substance.
Lynch and Lantinus were disqualified at the 2008 Beijing Olympics for using a banned medication.
Lynch's appeal is the third case registered by CAS at its Olympics court.