"I wasn't quite sure of the nature of it. He was in so much pain and I needed to know where that pain was.
"After a while, he was able to say it was near his lower breastbone, so what you think is the worst case scenario - the sac that covers your heart is behind there and you can bleed into that from a trauma, collapse your lungs, break your sternum."
However, Duffy has made rapid progress since Friday.
"The news has been good, a lot has changed in 36 hours," said Dr Byrne.
"On Friday night, we weren't talking about football, we were talking about would this young boy survive, would he live?"
Everton chairman Bill Kenwright and manager David Moyes have been in close contact, and dispatched their own medical staff to Dublin on Saturday.
Republic assistant boss Marco Tardelli said: "We are very happy now we know his condition. It was very emotional when I met his father because I have a son the same age as Shane."
Duffy's father Brian told the BBC that he "knew there was something wrong when Shane didn't get up".
"There was serious concern that we could have lost Shane in the ambulance - he was in severe pain.
"I should have been burying my son as the doctors and professors say they have never heard the like of it.
"If it wasn't for the work of the physio, doctors and nurses, Shane would have been dead.
"They say it will take him six weeks to recover, three months until he is back in light training, and that he should be back in full flight by Christmas time."
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