A young boy was spotted standing inside a rhinoceros enclosure in the Dublin Zoo on Saturday (July 30), accompanied by a male relative who was seen putting him inside.
Bystanders told The Daily Mail the man was prodding the boy to go further in the enclosure and pose for a picture.
They said the boy stood inside for about 20 minutes.
One told the Irish Daily Mail: "I didn't know the people. I only saw the man putting the child in the fence. The child didn't look scared and nobody really passed any heed on what he was doing.
"The child stood there for a good while and the man kept letting go of his hand and telling him to move further back to pose for a picture."
Another witness told the Irish Mirror: "It was such a stupid thing to do. He was only metres away from the rhinos. People were totally horrified."
One posted a picture of the man holding the boy's hand while he stood in the enclosure.
The picture shows the rhino and boy facing each other from a distance.
The Dublin Zoo said it was investigating the incident.
In 2006, a Siberian tiger at the zoo attacked a 16-year-old girl who stuck her hand through a wire fence separating her from the animal.
She was severely injured after the tiger attacked her.
Earlier this year in May, an incident at the Cincinatti Zoo in Ohio made headlines around the world when a four-year-old boy fell into a gorilla enclosure.
A 200kg gorilla dragged the boy around for about 10 minutes before it was shot dead.
There was an outcry over the death of the gorilla and questions raised about safety features in place at zoos.