Cathal Daniels was forced to withdraw his mare Rioghan Rua from Friday's Olympics dressage opener due to veterinary reasons.
Galway native Daniels will be replaced by Cork’s Austin O’Connor, who travelled to Tokyo as Reserve/P Alternate.
O’Connor will ride 11-year-old grey gelding Colorado Blue, who he owns along with the Salty Syndicate.
The Irish eventing team will not incur a penalty for the substitution as it takes place before the competition gets underway.
Daniels, who rode Rioghan Rua when helping Ireland win team silver at the 2018 Tryon World Equestrian Games, will revert to being the Reserve/P Alternative combination.
He could be called upon later in the competition if the veterinary report improves.
Irish eventing team manager Sally Corscadden said: "Obviously this is really tough for Cathal and for Rioghan Rua's owner and breeder Margaret Kinsella, particularly with it happening so close to the start of the competition.
"We did everything we could and gave the mare every chance to be ready but unfortunately we were forced to make a decision and of course horse welfare is always paramount for everyone.
"Austin now steps in and he is ready to go. We are very lucky to have someone of Austin’s calibre to step in, he is in great form.
"We still have a very strong team and are looking forward to the start of competition."
Carlow’s Sam Watson and Tullabeg Flamenco are the first Irish combination to take part in the dressage stage of the eventing competition on Friday (1.46am Irish time).
O’Connor and Colorado Blue will perform their dressage test at 11:24am while Meath’s Sarah Ennis and Horseware Woodcourt Garrison will start at 2:32am on Saturday.
