Once you leave Ireland you're written out of the narrative. The moment your passport is stamped you're lost to history.

Those first two steps you take onto the 767 that will carry you overseas are very fateful ones, because whether you know it or not you've just crossed an invisible line that says your story no longer matters to the land of your birth.

It's a hard thought, but you'll discover it's a true one. When you finally do return you might as well be Oisin coming back from Tir na nOg (the mythical Land of the Young).

People will remember you, but in the way they remember the lost summers of their youth. You become an echo rather than the thing itself.

They talk to you about the person you were, not the person you are. There won't be a thing you can do about it.

I saw what happens to all those haunted echoes all over the stage of Eugene O’Neill’s "Long Day's Journey Into Night," now playing at the American Airlines Theatre on 42nd Street. Onstage a brilliant Irish American family tries – and it must be said mostly fails – to make sense of the legacy that forced emigration and a lifelong struggle in the new country have handed them.

The severances of the 19th century were especially cruel, of course. The Great Hunger, the evictions, the grinding poverty of most Irish people under British rule, contributed to a longstanding era of crisis and collapse that left many with no option but to leave their homeland.

Arriving in the Five Points or in other new American ports of call must have felt like they had landed on Mars. Their language, their traditions, their whole sense of themselves must have undergone a radical and irreversible shift in order to survive in the new world.

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So we inherited, on both sides of the Atlantic, the shared post traumatic stress of a cruelly exiled people – from those who left and those who were left behind – but we have never honestly responded to it with the imagination and sensitivity that it clearly required.

That's because internally we saw much of it as our own failure, this breaking down, this taking flight. Many of us were secretly shamed by it, we blamed ourselves or others.

In Ireland, instead of responding to psychic wounds that British colonialism presented us, we simply pathologized it as a national failure and ran in the other direction: be it mental health, single mothers, gay people, the poor, travelers, immigrants, we have long considered all of them to be emblems of a wider social misfortune, fates that befell others, and which they may have deserved, that were to be stepped around and avoided by ourselves.

The unfortunate could infect the fortunate, history taught us, so they were to be avoided at all costs. They didn't bear thinking about. Lock the door and throw away the key.

The Irish reflex, shaped by centuries of collapse and severance, often tells us to fear the very things that can make us whole. O'Neill understood the cost of this tragic severance more than any other Irish American writer, and so his haunted Irish family must share the stage with a host of unquiet ghosts.

“An only life can take so long to climb clear of its wrong beginnings, and may never,” wrote the poet Phillip Larkin, in a line that encapsulates what the O'Neill's onstage and offstage family contend with.

Climbing clear requires facing up to the pain of the past, and that's a thing O'Neill's drinking and drugging characters continually refuse to do.

O'Neill's father came from Kilkenny. He was born there in 1847, the darkest year of the Great Hunger. He became famous and rich in America.

But there isn't a statue, plaque or even an inscription in his honor to be found in Kilkenny to this day. Gone and forgotten about.

But he carried Ireland with him (good and bad) and he passed it on to his son, who spent his gifted life struggling with that inheritance.

O'Neill's genius was to see what others had refused to. He saw the true cost of enforced exile, the half ghosts that were sent out on the roads and could never find a home again.

He also saw that once lost, Eden stays lost. You can't go back, but he found that you can't go forward either until you come to terms with your own hard severance.

The Irish don't think about you after you leave because there's an implicit criticism in your absence that they prefer to avoid. But cutting the chord that connects your experience abroad to the old homeland is the most short sighted thing they ever do.

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The moment your passport is stamped you're lost to history.