Cian O'Connor has lost his Olympic gold medal after the sports ruling body found that his horse had banned substances in its system during the games.
After the 12-hour hearing the International Equestrian Federation disqualified the rider from the Athens games and banned him for three months after finding he breached medication regulations.
However the FEI said it was satisfied that Cian O'Connor was not involved in a deliberate attempt to influence the performance of the horse.
Mr O'Connor said he was disappointed and his solicitor refused to rule out an appeal.
O'Connor, who gave evidence at the hearing, denied any wrongdoing.
A four-member judicial committee of the International Equestrian Federation heard the case. The 25-year-old showjumper was represented by a legal team including two senior counsels.
On the night of his victory in Athens, two drugs normally used by humans were found in a urine sample taken from Waterford Crystal.
The horse's B or second urine sample was later stolen in the UK but examination of the B blood sample confirmed traces of the banned drug normally used as anti-depressants by humans.
Last November Mr O'Connor's solicitor said the drugs were given to the horse by his vet for therapeutic reasons only.