Even today, surveying the grim statistics of the First World War on this anniversary of the battle of the Somme, I am still surprised by the scale of the Irish involvement in that terrible conflict.
Some 40% of the adult male population on the island of Ireland 270,000 men fought with the British army on the Western Front.
Two hundred thousand served in Irish regiments of the 10th and 16th Irish Divisions or the 36th Ulster Division. Many thousands joined other parts of the British army and still more worked in munitions factories in England.
The sheer human impact of these numbers is hard to imagine there cannot have been a community or family in Ireland untouched by the war.
Most of these soldiers were volunteers there was no conscription in Ireland and no doubt their motivation for fighting was varied and complex.
These were men with different political ideals and none, of different and opposing religious traditions, with radically different feelings of cultural and national identity. Yet their experience of the war was often a shared bewilderment at the carnage, and shared tragedy and sacrifice on the battlefields of the Somme and the Messines Ridge, Ypres and Verdun.
The Irish Division alone lost nearly 5,000 men at the battle of the Somme. Guillemont-Ginchy, where they fought, is now a quiet village with a modest Celtic cross as a war memorial where I and many others hope to lay wreaths later today.
That we and friends from the Irish Republic are able to honour these men, alongside those who fought in the Ulster Division, is an important and relatively recent development. It has, I believe, lessons for all of us engaged in the debate about how to deal with Northern Ireland's troubled past.
For understandable reasons the political turmoil in Ireland which followed the Great War led to an ambivalence about Irish soldiers returning from France.
Against the background of the Irish war of independence and the civil war, the traditional armistice day commemorations became unsustainable and even the memorials fell into a state of neglect.
For the soldiers themselves many of whom returned to disillusion about the policy of the British government for which they had volunteered the experience of returning to a place where their sacrifice could not be recognised must have been doubly disillusioning.
In the decades which followed, the year 1916 would inevitably be remembered for the Easter Rising and not for the Great War, despite the huge numbers caught up in the tragedy of the Somme.
The revival of interest in remembering the Irish dead of the First World War during the 1980s and '90s was itself a sign of healing and of the dramatic change in relations between Ireland and Britain.
The renovation of Islandbridge memorial in Dublin and the opening by President McAleese of the Irish Peace Park at Messines were confirmation of that radical revaluation of a past which had been something of an embarrassment.
There are some obvious lessons here for all of us emerging from a very different sort of conflict and trauma over the last three decades.
First, we have to accept that the passage of time plays a role, although I hope it will not take 60 years to reach a consensus on how to deal with the past in Northern Ireland. But, more importantly, we have to accept that the past is not easily detached from the politics and the mood of the present in short, it is not easy to decide where the past ends.
Nowhere is this more true than in Northern Ireland, where historic conflicts fuel present tensions and present bitterness infuses all attempts to address the past.
There are no easy answers, but perhaps one key to unlocking a process of reconciliation is to remember the individuals who fought at the Somme.
Anyone who has taught history, as I have, is accustomed to presenting the Great War as a succession of 'great' events and personalities, of international politics and, in the Irish context, of the great debates over Home Rule and independence. But the history of the Great War is above all else the human story of individuals, from families across Ireland, Catholic and Protestant.
It is the story of remarkable suffering and terrible grief. One does not need to make value judgments about the declaration or the conduct of the war, or the motivation of its soldiers, to appreciate that.
It is this focus on the victims and their suffering, on what the Great War poets called 'the pity of war', that offers the best hope for coping with our own past in Northern Ireland.
It will not be an easy process and we have barely begun. But as I look out across the endless rows of war graves in Flanders I will know as every visitor does that however terrible the suffering and however deep the bitterness, reconciliation is both possible and essential.