He was 14 years old. So he said. "People like you, bothering 14-year-olds - you're why murders happen."

I'd boarded the Dart mid-morning on a nearly empty carriage, towards the back, to be near the stairs when the train stopped at Grand Canal Dock. I sat down, only then realised the noise wasn't stopping, wasn't a ringtone.

Peeking over the top of the worn green fabric seats was the top of a white waterproof hood with 'Columbia' stencilled in red. A white trainer and blue-tracksuit-bottomedankle pressed against the window. And on the tinniest of speakers from a cheap mobile, the worst awful nightmare of dance music that wouldn't have sounded good on a Saturday night after three ecstasy tabs and a table full of vodka and red bull.

Implausibly loud. Maybe because the carriage was nearly empty. Maybe because I just wasn't in the mood.

I popped in headphones to try and drown it out, looking out the window; but finally wanted to punch out my own ears with a knitting needle from the old lady across the aisle. Then I looked up and realised we'd gone just two stops, to Sandycove and Glasthule. There was no way I could stand the entire journey; like I'd fallen asleep and woken up on an F train to Ave X in Brooklyn in 1988. I moved away from New York because of crap like this. I moved to London, then Dublin. I moved out of Parliament Street to the ridiculously charming idyll of Dalkey, at great expense, because the 4am wake-up call of fights over kebabs and birds taking place three floors below had lost its charm.

It's the Broken Window Theory. Gospel of Giuliani as retold by the Prophet McDowell. Let them play loud music in a peaceful morning suburban rail carriage and next thing you know you've got crack dens and drive-by shootings. Someone had to step up. Full of self-important righteous dignity (somewhat marred by the iPod nano hanging on a lanyard around my neck) I stepped into the Hoodie's field of view.

"You mind turning that down?"

"Wha'?"

"Down. Would you turn that down."

Blinking incomprehension.

Then: "Whaya gonne do f 'I don'?"

"Well, I might throw it off the train. Maybe with you still attached."

Now he sat upright. He switched it off.

"You. Gonna take this. Off of me."

"Just turn it down. Please." I sat down.

He turned around, sized me up. He judged. He switched the noise back on.

Started singing along, eyes not quite focusing. "Whaya gonne do, businessman? I'm jus listening. Nobody else have a problem with it.

Nobody else ask me turn it off." He glared up and down the carriage. No one looked up.

I suggested we take a vote.

He'd go around and ask each passenger if she minded. If everyone else said they didn't, I'd shut up. I just smiled, didn't turn away. Then out came the screwdriver, him looking straight at me, pressing it to his own neck, the flesh yielding at the jugular. I couldn't help myself. I laughed. "You going to unscrew some bolts - maybe derail the train?"

Jesus Christ, it was 10.30 on a weekday.

"What're you? Businessman?"

"I work for a newspaper."

"In the factory, like?"

"No." Pause. "What are you going to work at?"

"Carpenter." Starting the tool collection early. Outstanding.

"Why?" I asked, not sure why I did, unaccountably picturing the screwdriver going through his wrists pinning him to wood. Like another carpenter. "It's good work.

What newspaper you work for?" I told him. "Do you know [X]? I race motorbikes. He'd know me from tha."

A class of kids got on then.

No older than six. Flooded the train with chatter. End of conversation.

Hoodie stood up a few stops later, paused as he walked by. "Take it easy, " he said. "You too, " I said. Meant it, too.