All is changed, changed utterly," wrote William Butler Yeats. "A terrible beauty is born." He was reflecting on the Easter Rising of 1916, but his words, in a very different context, were just as appropriate yesterday as a British monarch stood in Dublin's Garden of Remembrance and paid homage to Ireland's dead.
Nearly a century on, all is indeed changed, changed utterly on this island in the relationship with its nearest neighbour.
Had Yeats been there yesterday, he might have observed a new beauty born in the Queen's historic handshake with the Irish president.
For me, and for many others of Northern Ireland stock, that handshake was the fulfilment of a lifetime's hopes.
None of us believed we would live to see the day, but now it has come and passed and the sun and the moon and the stars are still in the sky, the grass is as green as ever and an invisible and imaginary bridge has been constructed overnight which spans the Irish Sea.
Though Dublin's streets were silently surreal - the visit sanitised by security precautions - nothing could take away from the symbolism of the Union Flag and the Tricolour fluttering side by side, the British and Irish anthems echoing over Phoenix Park and the scene in the Garden of Remembrance as Elizabeth Windsor and Mary McAleese stood together and laid their wreaths.
"Some men see things as they are and say, 'Why?'," wrote the great Irish playwright George Bernard Shaw. "I dream things that never were and say, 'Why not?'"
Was yesterday a dream? At times, it seemed as if it were, if not a dream, then a world of make-believe.
From the moment the Queen's plane appeared through the grey skies over Casement Aerodrome and taxied to a halt, with her standard fluttering from the cockpit window, the dream was real.
There she stepped on southern Irish soil, dressed in tactful emerald green and St Patrick's blue, a diminutive, aged and arched octogenarian, carrying the full weight of Anglo-Irish history on her shoulders.
There she stood outside Aras an Uachtarain and before the Garden of Remembrance memorial, the intensely emotive strains of A Soldier's Song and God Save the Queen echoing in her ears and across the green grass of Phoenix Park and over the rooftops of Dublin.
There she stood alongside the Irish President, two women so far removed from one another by birthright, but now in common cause to rebuild respect between their countries.
No: the Union Flag and the Tricolour side by side was no Shavian dream. Nor was the 21-gun salute which opened on the first bar of the British national anthem and ended on the last bar of the Irish one.
Irish nationalism did not look threatened or diminished in any way by the presence of the Queen in the Republic. On the contrary, it seemed to be enhanced by every step she took.
If only the protesters had opened their eyes, they would have seen that this was a victory for the Irish in every way.
It was the day the Republic could show off full, independent republican credentials and, in the welcome afforded to the Queen, consign more of our divisions - unionist and nationalist alike - to the past.
The events of this week mark the start of a new journey and none of us knows where it may take us finally.
All is changing utterly, north and south, because there is a belated recognition that our futures, British and Irish, are intertwined - whether we like it or not.
The Queen took only a hour to cross the Irish Sea yesterday, but like President McAleese she travelled far further in the time machine of history.
I recall being invited to Mrs McAleese's inauguration as President in St Patrick's Hall in Dublin Castle, where she is hosting a state banquet for the Queen tonight.
Many unionists were wary of Mrs McAleese, who held a high moderate-nationalist profile in Northern Ireland at Queen's University.
Perhaps it was a strong hint of the route the new president planned to travel that more than a sprinkling of northern faces - and not all from a nationalist background - were in St Patrick's Hall that night.
The Queen's visit should help to underpin more respect for unionists on this island.
We have come a long way from the days when the mood of many in the Republic seemed to want to drive the Brits and even unionists off these shores.
My lifetime spans the post-war years and the Queen's long reign. The first time I watched television was to see the grainy, monochrome images of her Coronation.
We crowded into our primary school classroom in Co Tyrone on June 2, 1952 to view the ceremony and afterwards were each given a Coronation spoon.
Soon after, a young Queen Elizabeth appeared in person in my home town's Market Square, driving past us as we waved miniature Union Flags, which we had been given to us specially for the occasion.
All of this experience was exclusive to the unionist community.
For much of my life, the Queen has only been a unionist queen, ignored and treated with total indifference by the nationalist community.
Now her visit to the Republic is the Irish president's crowning glory and has helped undo centuries of enmity between England and Ireland.
Yesterday's street protest and the lack of contact with the public shows that there is still some way to go to total normalisation of relations with Britain.
However, the speed of change has become exponential in the past decade.
The dream has become real and yesterday's historic events mean life has changed utterly for all of us on this island in May 2011.