'You can't go back to the country that doesn't exist anymore'

How, then, we have changed: it is a bright afternoon in March. The sky above New York is blue as birdshell. I am walking with the Irish novelist Sebastian Barry across Brooklyn Bridge. We have no other reason than to clear our heads. To chat. To walk. There is an isolated grace to walking in New York. Everyone else seems to be moving to a purpose and we, the walkers, seem to belong to the very air of the city, as if we have become particles that float, along with every other particle in the city. It is a Whitman feeling. The wooden footpath beneath us trembles. Far below, a flash of grey from the East River. The skyscrapers of Manhattan seem Easter Islandish. The Statute of Liberty in the distance. I can think of nowhere else in the city that can be so crowded and yet so quiet at the same time.

We are walking beneath the harpsicord of steel cables, Sebastian and I, when suddenly a bicycle appears in front of us, an old-fashioned banger, cutting through the bright blue of the day. The cyclist, a woman, is going along a good clip. Silvery hair blown back, her cheeks red from the wind. A satchel in the front basket. We step apart while the bicycle goes through. "Nuala!" She pulls hard on the brakes, puts her foot to the ground, and turns. Nuala O'Faolain. It is one of those moments, when surprise is the key, when the oxygen is taken from the air, when the world is turned momentarily sideways, when the first thing that is said will be the thing that is said forever.

"My damn brakes," says Nuala, "they aren't working at all."

There is a Portugeuse notion of "saudade," which is that person or place that draws out of us our most extreme or improbable yearning. It is a feeling for something which is gone but might return, a vague desire for something other than the present. Saudade is different from nostalgia in the sense that what is longed for might, at some stage, return. It often hits us at the most peculiar times, a need for presence as oppossed to absence.

I grew up in Dublin and Grafton Street used to be the place where I'd run into people I knew. I'd be dawdling along, coming from the Dandelion Market, or up from the end of Nassau Street, or sideways from Davy Byrnes, and inevitably a few faces would come out from the crowd, people I knew, who'd stop and chat and from whom I'd borrow, beg, plead, steal. But it was a small city then, Dublin, or so it seemed, full of character and characters. It seemed to throb with possibilities and recognition. I thought I'd never lose it.

Recently I walked along Grafton Street and didn't know a soul, hardly recognised a shop in the city where I grew up. I had lost that auld Dublin, no longer recognised it for what it used to be. I felt homeless, sad, bewildered. Maybe it was just a sign of middle-age, something I had to accept I was a good thirty years out of my short pants and perhaps it was time to stop being sentimental. Joseph Brodsky once said: "You can't go back to the country that doesn't exist anymore."

But then it happened, just the other day, when Nuala O'Faolain complained of a ticking sound from her green bicycle, and Sebastian Barry, with a keen eye for what is wrong, suggested that it was only the kickstand, and then we leaned down to fix the bicycle – a bicycle with a basket! – and together we stood in the raw wind off the East River, and talked literary shop, as if it was the most natural thing in the world, the very pulse of the ordinary, that we were on Brooklyn Bridge, in New York, three Irish writers, as if we have met there every day, and it struck me then where home was, and I had that moment of saudade, for Grafton Street and all that had been left behind, that gone life, and yet a moment of great relief too for I knew that Grafton Street was suddenly everywhere, even over the East River.

Nothing refreshes quite like a tinge of loss.

Joyce, in a letter to Frank Budgeon, once said that he had been so long out of Ireland that he could hear all at once her voice in everything.

We watched Nuala go towards the Brooklyn side, a high wave in the air as she gathered speed, and then Sebastian and I took the sloping walkway down towards Manhattan. I have spent so much of my life apologising for having left Ireland, and it was at that moment, when I could hear the thrum of the traffic from the down near City Hall, and the whole of New York's jagged toothline was spread out in front of me, and the quick zip of yellow taxis went along the concrete shoreline, that I realised I have never really left Dublin at all.

Colum McCann's column for Round Midnight with Donal O'Herlihy on RTÉ Radio 1 (11.40pm) goes out every Tuesday night