A teenage girl showed "no pain" as she danced naked among shards of glass on the floor of a house after taking designer drug 2CB.
The 18-year-old was covered in so much blood that it was "impossible to tell" if she had any wounds to her body.
She was "so out of it" that she failed to realise an 18-year-old man was lying on the floor having suffered a suspected heart attack.
He is in a critical condition and three others remain in hospital after taking 2CB.
The potentially deadly ecstasy substitute was taken by young people at a party in the Greenmount area of Cork city.
Gerard Banks, who was first on the scene, said he was walking home when he heard "crazy shouting" coming from inside the house.
When he peered through the front window, he saw blood on the walls, floor and curtains.
Speaking to the Herald, he said a man who was "clearly on drugs and in a state of shock" then appeared at the window.
"We asked, 'Is everyone OK?' He replied, 'Everything is fine'," said Mr Banks.
"But when we got into the house there was blood everywhere. Those who were in the house were not in control of themselves at all. Some were injured and soaked in blood from head to toe."
Mr Banks said the main objective was to try to "keep everybody under control" until paramedics arrived.
"The whole house was smashed up. There was glass on the floor from a broken mirror. They were just walking around on the glass," he said. "They didn't know what they were doing. That was the scariest bit ."
What struck him most, he said, was the amount of blood.
"It was everywhere," he said. "They were shouting and dancing around the house while a guy was lying on the ground."
Mr Banks said members of the emergency services were shocked by what they saw.
"There was a girl in the house who looked about 18 and she was completely naked. She wasn't with it at all. She was covered in blood, but I couldn't tell if she was hurt," he said.
"There was also a man lying on the pavement outside the house."
Gardai have arrested a 29-year-old man in Cork follow- ing a series of raids on Tuesday night.
Officers raided two addresses and discovered traces of the 2CB drug consumed by six students in the city.