He was pictured tumbling off his bike during the event in Dublin after a fallow buck tried to leap clean over him.
The triathlon was underway at Phoenix Park in Dublin City.
Shane O’Reilly told Irish broadcaster RTÉ that although his helmet was cracked, he managed to carry on and finish the race.
‘The helmet definitely saved me,’ he said. ‘Apart from that, my shoulder was a bit sore and I had a headache.’
Despite the unexpected collision, O’Reilly still completed the race in two hours and 25 minutes.
Photographer Erik Scraggs, who took the photos, said he was totally taken aback by the deer’s attempted leap. Fortunately, both the cyclist and the deer didn’t suffer any injury.