By Kevin Connolly
Ireland correspondent, BBC News
The 36th (Ulster) Division fought on the early days of the Somme
Irish soldiers played a major part on the Somme during World War I.
Their involvement had repercussions for Ireland long after the initial fighting of the battle.
The Irish troops were committed into the Battle of the Somme haphazardly alongside the English, Australian, Welsh, South African and Scots and other Imperial forces - not to mention the French, and French-African troops who fought alongside them.
Still, it is possible to follow the Irish thread through the confused tapestry of the fighting.
The 36th (Ulster) Division was committed in the attack on the first day, tasked with taking a German fortification called the Schwaben Redoubt.
They were among the few units to reach their objective, but reinforcements despatched into the carnage of no man's land never reached them, and eventually, isolated and surrounded they were forced to retreat.
Of the nine Victoria Crosses awarded on the day, three went to the Ulster Division - two of them posthumously.
The Division was relieved on 2 July having suffered more than 5,000 casualties - 2,069 of whom were killed.
Tattered and traumatised, the Ulster Division withdrew from the battlefield to re-group and march directly into the political mythology of Ulster Unionism.
Their "blood sacrifice" was seen as Ulster's side of a deal in which Britain would somehow "see the loyal province right" in the agonising over Home Rule which was sure to resume when the fighting was done.
Their legend lives on. One of the Protestant paramilitary organisations in modern Northern Ireland uses the title Ulster Volunteer Force precisely because of the historical resonance they know that title has for northern Protestants.
Images of the old volunteers are still to be seen in the banners of Orange lodges and in the huge murals that adorn gable ends in working class areas of Belfast.
It is worth bearing in mind that the annual Orange march at Drumcree in County Armagh, whose route remains a subject of intense political controversy to this day, is a commemoration of the first day on the Somme.
There were, of course, Catholic soldiers from the south of Ireland in the fighting on that first day - the Royal Dublin Fusiliers amongst them - but we pick up the story of the Irish at the Somme in September.
The front line - a huge metallic scar through pretty French meadowland - had barely moved since July, although the casualties on both sides had been beyond imagining.
More than one million men would be dead, injured or missing by the end of the fighting and many of the injured were permanently disabled.
On 3 September, another great British offensive went in. This time the soldiers included the mainly Irish Catholic 16th Division, brought down from Loos in Belgium.
Their objectives were the hamlets of Guillemont and Ginchy, which the original battle plans had assumed would be taken in the first few days of fighting.
The men of the 16th Division fought with the same reckless courage that had distinguished the 36th Division - their sacrifice separated only by a few months, a few miles, and hundreds of thousands of casualties.
Within 10 days the Division had lost half its 11,000 men killed or injured. Most died anonymously which was the way of it in a war which married the tactics of an old century with the technology of the new.
Not all though. A private soldier in the Connaught Rangers and a young officer in the Leinsters won Victoria Crosses.
The manservant of the poet and nationalist MP Tom Kettle, who died, wrote movingly to his wife: "He carried his pack for Ireland and Europe. Now his pack-carrying is over. He has held the line."
The problem for those Irish nationalists like Kettle who served was once the war was over, it was difficult for them to define the cause for which they had fought.
Ulster Protestants returned home, vindicated and demanding. They knew what they had fought for, and they knew what they wanted in return.
If Kettle had lived he would have returned to an Ireland in which the political initiative had been seized by nationalist rebels who refused to fight for Britain, and indeed staged the Easter Rising just two months before the assault on the Somme began.
It was the handful of men who stayed at home to fight the crown, not the many thousands who went to Europe to fight for it, who took control of Ireland's political destiny.
They took control of Irish history too, and it was the fate of men like Kettle to be airbrushed out of it.
The Battle of the Somme ground on for another two months or so, and eventually petered out in the rains of November.
It was not obvious at the time, but the Somme would fix itself in the popular imagination as a kind of metaphor for the Great War.
The British and Allied armies suffered 420,000 casualties to move the front line just a few miles in four-and-a-half months.
And Ireland? Well, Irishmen North and South joined up in the hope of somehow advancing their different causes, unionist and nationalist, in fighting on the British side.
There is no space here to argue over how they succeeded and how they did not - but it is worth noting that for all that has happened since 1916 the echoes of the same issues which divided Ireland on the way in to the Great War continue to divide it now.