The first time I met Aisling Kelly she was wearing an old-womanish cardigan, mossy-green and fastened with oversized plastic buttons, and a skirt tied with a broad leather belt. She was carrying her folder of school notes, each section alphabetized and tagged with green or pink labels.

‘What do you think will come up on Biology Paper Two?’ she asked me.

Stephen broke up with her after two months.

‘Did you fuck her, man?’ Fergal Morrison asked him, two weeks later, voicing the question that was on everybody’s mind.

Stephen silently held up his hand for the high five.

Two weeks after they had broken up, Stephen sent Aisling a text message that said, GUESS WHO I GOT A BLOWJOB FROM LAST NITE? HINT, IT WASNT U!!!!

A fact about Stephen O’Brien that never made it into any of the newspapers: he was worried about losing his hair. Maurice O’Brien was bald except for two grandfatherly tufts of grey hair at his temples. Stephen’s hairline was receding noticeably by the time he was eighteen. It was something he mentioned only to his closest friends, and only in moments of real intimacy.

You didn’t slag Stephen O’Brien about his hair. It just wasn’t done.

Stephen didn’t look like a violent man. He looked like a strong man – you took his strength for granted. But you wouldn’t have marked him for a killer.

A few days after Stephen had broken up with her, Aisling Kelly complained to her friends that Stephen didn’t know how to put a condom on correctly. ‘He doesn’t pinch the little bit at the top. You know what I mean? He just, like, rolls it on. There was no fucking way I was getting an abortion for Stephen O’Fucking Brien. So I told him to cop on and leave me alone.’

People began to say that it was Aisling who had broken up with Steve, instead of the other way around. Nobody at Brookfield believed these rumours.

Maurice and Kitty O’Brien were the prosperous children of, respectively, a doctor and a lawyer. Trinity-educated as undergraduates – two years apart – they had met when they were both pursuing graduate degrees in Business Administration at Oxford. They owned a three-story house in Ranelagh that they had bought and fixed up in the late 1970s. Maurice was a senior executive in the Irish branch of an international software development company. Kitty owned and operated a beauty salon in Donnybrook. Eileen Harris and Katherine Culhane regularly went to this beauty salon, where Kitty O’Brien gave them a discount.

Is it significant, I wonder, that all three boys who went to prison for killing Conor Harris were only children? Does it make a difference? Historically, of course, the middle classes have always had fewer children than any other class. But these were Catholic families – Irish families. Why did the Culhanes, the O’Briens, and the Foxes not produce more than one child each? Was this a contributing factor to what happened on the last night of summer, 2004?