This one-time member of the Ulster Defence Regiment is close to the family of Eva Martin, a 28-year-old UDR soldier killed in 1974 in a mortar attack by an IRA team that included the 19-year-old O'Callaghan. Like many of the retired and serving Northern Irish police who knew Sean had murdered Inspector Peter Flanagan that same year, yet who came to St Martin-in-the -Fields in Trafalgar Square to honour him, he admired his courage.
Having realised he had been a foot-soldier in a squalid sectarian war, Sean had atoned for his crimes on a grand scale by risking his life as a highly-effective Garda agent within the IRA - unpaid at his own insistence -then giving himself up to serve time in jail and spending the rest of his life fighting violent ideologies.
Ulster Protestants see repentance as a prerequisite for forgiveness, which most of them give in abundance if they feel it deserved. For a community persecuted and demonised for decades by terrorists who regret nothing and lie about the past, Sean O'Callaghan is a hero who sought and achieved redemption.
The St Martin Vicar, Dr Sam Wells, wrote that he had done something new: "In my prayers, at a memorial service, I remembered not just the deceased, his children, and friends; but the two people he murdered." It felt, he said, "like the whole of the Irish tragedy in one hour - in one life".
That tragedy included enforced exile from his beloved Kerry. Following on Sean's brother's reading of Patrick Kavanagh's powerful Epic, which speaks to the love of one's home place, Professor Liam Kennedy told us how Sean could follow his county's triumphs in Gaelic games only through news reports.
Colonel Tim Collins - brought up in Belfast during the Troubles - told the press: "Sean, I think, in his latter stage became an Irish statesman, and because he had come from inside violence he was the best person to show us the road away from violence. He also identified and uncovered Sinn Fein and the IRA for what is was - one and the same organisation - and for that I think everybody should be grateful."
Charles Moore, ex-editor of the Daily Telegraph, explained the extraordinarily eclectic group thus: "Like a true man with moral courage Sean... was the same with everybody, he didn't care whether they were rich, poor, famous, unknown, infamous." There were waiters, recovering alcoholics and many others who had known nothing of his past sitting beside journalists and politicians who had known him precisely because of it.
Arlene Foster was there, as was David Trimble, who gave a reading. Novelist Lionel Shriver rubbed shoulders with Mairia Cahill, niece of the IRA icon Joe Cahill. And this was the man republicans portray as a friendless Walter Mitty.
Addressing a congregation of around 300, the writer Douglas Murray said: "Sean was compartmentalised, keeping his many friends and family in different boxes, a habit born perhaps as a past necessity. Many people here today were unaware of each other's existence." It was noted also by the Marquess of Salisbury, who said this would be the only meeting of this particular club. "I will miss him," he said, "as we all will. I will miss his intelligence, his gentle teasing of allies and his capacity for original thought. Ireland has lost a true patriot and the United Kingdom a good, if not uncritical, friend." It was a point underlined by the historian Lord Bew in his reading of Othello's speech about doing the state "some service".
The Home Office Security minister was there and for the first time, through the attendance of the Irish Embassy's Political Counsellor, the Irish State acknowledged its debt. As Sean's son Rory said afterwards, "I feel that both the British and the Northern Irish establishment did honour to the duty and service that my father did to both nation states, both these islands, and the Irish government finally acknowledged the service and sacrifice he made on behalf of the Irish nation and peace on these islands."
I gave Sean the last word in a reading from his book on James Connolly, his onetime hero. "Connolly possessed the ugly sentimentality of the true absolutist, and only the selfless pursuit of his holy cause, the global Workers' Republic, that shining city on the hill, could satisfy the demon within. Ultimately it would kill him and, directly or indirectly, many others who had never heard of Karl Marx and had no wish to die for some utopian ideal. And it seemed to be a life mostly without joy. 'As long as I live I will have no rest,' Connolly declared, 'only working, educating, organising and fighting to destroy the forces that produce poverty.'
"As a young man I admired this kind of commitment. Now I find it ugly, sad and empty of what it is to be truly human."
Well, the admiration and love expressed last Wednesday certainly brought home that Sean had learned what is true humanity.
It's about loving your neighbours, not murdering them for the sin of being from another tribe.