Charley Cullen sprang to action when Finn, 79, blacked out behind the wheel as his car hit 73mph.
The schoolboy grabbed hold of the wheel and pulled the vehicle back on to the road, saving both his and his grandfather's life.
The drama happened in Co Meath, Ireland, as Charley was being driven home from a drama class.
Charley told the Irish Mirror: "My papa was picking me up from Rathkenny Hall – we were about 500 yards away from Slane Castle and the car pulled into the ditch.
"I thought it was an April Fool's joke because it was on April Fool's day. But when my granddad dropped his head and took his hands off the wheel I knew it was no joke then.
"I grabbed the wheel and pulled it back out onto the road. We met a few cars going by. I had one hand on the steering wheel and the other trying to wake papa."
Finn had blacked out but his foot was still heavy on the accelerator, racking up 73mph.
But amazingly Charley kept his cool and managed to successfully steer the car away from a busy main road.
Speaking to Brenda Donohue on RTE Radio One's Mooney Show, Charley – who fractured his skull in the drama - said: "The only way I had any experience was the Grand Theft Auto game.
"My main thing wasn't worrying about the car – the main thing was to wake papa up. I had a bit of adrenaline running through my body when I grabbed the steering wheel.
"Coming near Slane Castle I went to turn left to go to Slane Village because if I went right there's a big drop beside Slane Castle and a river flowing through it and I don't think I would have got out of that.
"I turned for Slane and we amazingly got across the main road with no cars coming.
"The car hit the wall sideways because we were turning for Slane.
"Before that I remember the car turning in the air and the light shining against the wall of Slane Castle and from that on I can't remember much."
The vehicle had crashed into the gates of the building but as Charley was still alert, they were able to get out.
He pulled his grandfather from the car and woke him up before carrying him 50 yards to his house.
Charley said: "I had a haematoma on my head and that night they were more worried if anything had happened to my brain.
"But a few weeks later I found out I had actually fractured my skull. Papa was bleeding a lot – we got to the house and my Nanna didn't know anything about what was happening.
"I opened the door and I can remember the scream in the back of my head."
Finn doesn't know why he blacked out but said it is a miracle both of them are alive.
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