Ireland was on the cusp of the Easter Rising when thousands of Irish men joined up to serve with the British army in the first world war. They returned to an utterly changed country; they were officially forgotten. This is one man's story

This is the story of my great-grandfather, of Ireland, and of the importance of remembering. Sylvester James Cummins was a carpenter, like his father. He was born in 1886 in the small market town of Bagenalstown in the beautiful Barrow Valley in County Carlow, an hour south of Dublin. The town is still known by its original Irish name, Muinebeag, which means small thicket, the term given for a dense stand of trees.

The English name came from the landowner, Lord Walter Bagenal, who modelled the town on Versailles in France. The Bagenalstown courthouse is based on that in Versailles, where the peace treaty between Germany and the Allied powers that officially ended the first world war was signed.

Sylvester enlisted with the 9th battalion of the Royal Dublin Fusiliers in September 1914. The majority of British first world war army service records were destroyed during the London blitz of 1940. Miraculously, Sylvester's papers survived, with char marks around their edges from the blaze. They contain information about his discharge papers, disability pension, disciplinary actions and signed receipts for his war medals.

His attestation papers are a reminder of an Ireland that no longer exists. A handwritten "Yes" is placed beside the question, "Are you a British subject?" Sylvester did not know it then, but Ireland was on the cusp of the 1916 Easter Rising, which would come to define the nationalist narrative of post-independent Ireland. "I, Silvester James Cummins, swear by Almighty God that I will be faithful and bear true Allegiance to His Majesty King George the Fifth, His Heirs, and Successors … So help me God," reads the oath.

But he did not sign it. Sylvester spelt his name with a Y, not an I. A glance at other attestation papers of Irish recruits reveals blanks in the oath, or a signature different to that elsewhere. It is a small thing, but I noticed it, and 100 years later that dormant nationalism still matters somehow. There is no logic in making this distinction. He wore a British army uniform.

Why did a southern Irish Catholic voluntarily enlist in the British army? John Redmond MP, leader of the Irish party, pledged to support Asquith's Liberal party in return for the introduction of Home Rule. The Ulster Volunteer Army, based in Protestant-dominated Northern Ireland, promised to use "all means that may be necessary" to prevent Irish self-government. The Irish Volunteers in the south were also ready and armed. Ireland was on the brink of civil war at the outbreak of the first world war.

Redmond made a pivotal speech in Woodenbridge on 20 September 1914, two days after Home Rule had passed into law and six weeks after Britain declared war on Germany. With Home Rule on the cards, he pledged his support to the Allied cause and urged the Irish Volunteers to join the British army, proclaiming that: "The interests of Ireland – of the whole of Ireland – are at stake in this war." Of the 80,000 that enlisted in the first 12 months of the war, half were from Ulster and half from the south. Sylvester enlisted five weeks after Redmond's speech.

Others enlisted for adventure, "for no other reason than to see what war was like, to get a gun, to see new countries and to feel like a grown man," in the fiery words of the future IRA leader, Tom Barry. Poverty also featured. James Connolly, the socialist revolutionary, contended that "economic conscription" attracted a large number of recruits from the improvised tenements of inner-city Dublin. In Sylvester's case, his father was dead and his army pay was sent to his mother and young sister.

Under the command of Tipperary man Major General William Hickie, the 9th battalion of the Royal Dublin Fusiliers of the 48th Brigade and 16th (Irish) Division of the British Expeditionary Force was mobilised for war on 18 December 1915. They landed in the northern French port of Le Havre the next day, and spent three years on the western front.

A recruitment poster in Ireland during the first world war. Photograph: Buyenlarge/Getty Images

Life in the trenches, shelling, charges into no-man's land and poison gas for ever scarred those who survived. "The bursting shells threw up earth that descended in showers, shrapnel and other shells came roaring along … There is nothing for you to do except to keep a firm grip over everything and wait till the bombardment stops." This letter was from a solider who fought with Sylvester, Second Lieutenant Bernard Reid.

Sylvester was involved in three major operations. He was in the trenches at Hulluch near Loos on the western front when the Germans launched a gas attack on 27 April 1916. The Irish Division suffered heavy casualties, with 538 dead. Hundreds more were to suffer chronic lung conditions for the rest of their lives. "I had the sad job of collecting and burying the dead," Lieutenant Lyon of the 7th Leinster regiment wrote. His description of his countrymen is heart-breaking, "some of them holding hands like children in the dark". This was the same day that news reached my great-grandfather and his battalion that the Irish Easter Rising had started.

Patrick Pearse had issued the Proclamation of the Irish Republic on the steps of the General Post Office (GPO) three days earlier. Thus began an unstoppable series of events that cumulated in the Irish war of independence of 1919-1921. A terrible beauty was born. "Irishmen!/ Heavy uproar in Ireland/ English guns are firing at/ Your wifes and children!" read the German placards opposite the Irish trenches. The Ireland Sylvester left would prove to be unrecognisable from the one he returned to.

The Royal Dublin Fusiliers were involved in two stages of the battle of the Somme in September 1916. The village of Guillemont was captured and the heavily fortified German position at Ginchy was taken. Sylvester's battalion lost 66 men at Ginchy, including the Irish Nationalist MP Lieutenant Tom Kettle.

They helped capture Wytschaete in June 1917, the opening day of the battle of Messines. He was also involved in Langemarck at the Third Battle of Ypres. This major offensive in Flanders in 1917 attempted to break through the fortified German defences enclosing the Ypres Salient. The difficult, waterlogged conditions caused major casualities, and Sylvester's records suggest that he was injured in September 1917.

Two years on the frontline were rewarded with a transfer to the Labour Corps, a unit for men deemed physically unfit for normal soldiering, but not injured enough to be sent home. Sylvester was medically rated "B2", below the "A1" condition needed for frontline service. As a member of the Area Employment Company, he would have done salvage work within range of the enemy fire, sometimes for lengthy periods, adding to the mental anguish already accumulated from the gas at Hulluch and the shellshock from the trenches.

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We know it now as post-traumatic stress disorder, or PTSD. There was no understanding at the time of the psychological effect of experiencing intense fear and raw horror over a prolonged period of time. "This is not a war," Sebastian Faulks surmised in Bird Song, "this is an exploration of how far men can be degraded." By the time the armistice was signed in November 1918, on "the eleventh hour of the eleventh day of the eleventh month," Sylvester had survived two years of trench warfare and another awful year in the no-man's land of the Labour Corps.

The Irishmen who fought in the first world war were officially forgotten in post-independence Ireland. The end of the war coincided with a changed political climate. Redmond's call at Woodenbridge was rewarded with just six seats from 105 for the Irish party at the 1918 election. Home Rule was dead. The militant nationalism expressed by Éamon de Valera's Sinn Fein was in the ascendancy. All had changed, changed utterly.

In November 1920, Sylvester's disability pension was approved and he signed the receipt for his army medal, the 1914/1915 Star. This was the same month that my grand-uncle, no relation to Sylvester, took part in the assassination of British intelligence officers on what became known as Bloody Sunday. Charlie Byrne was a member of the revolutionary leader Michael Collins's "apostles", an elite team of men with the specific purpose of killing British armed forces during the Irish war of independence. Two Irishmen on different sides of history: one served in a British army uniform, the other killed men wearing them.

This was not an Ireland for a southern Catholic who had served in the British army. A collective national amnesia had decided that the southern Irish soldiers belonged neither to the unionist tradition of the north or the republican legacy of the south. Many veterans, including my great-grandfather, decided to live outside Ireland after the war. Poverty and high unemployment were certainly factors, but so too was the explicit hostility to those who had served in the war.

Sylvester Cummins, an Irish carpenter who served in the British army, taken sometime in the 1920s

"Let there be a war memorial. That is one thing, but a war memorial in Merrion Square, a public park, presumably with the railings gone and leading up to the entrance of Government Buildings, is another thing." The minister of justice, Kevin O'Higgins, was adamant that any memorial for those that had died in the war, including his own brother, would be out of sight and therefore out of mind.

It was not until 1988 that the Irish National War Memorial Gardens, 5km from parliament on the outskirts of Dublin, were formally dedicated and opened to the public. The Queen's visit to the Islandbridge memorial in 2011 was the first time that I became aware it existed.

The first world war was not taught in Irish schools. Most Irish people would be surprised to learn that an estimated 200,000 Irishmen served in the British army. The silence in our history books about the 50,000 dead is sorrowing. Many other Irish-born men fought and died with the American, Canadian, Australian and New Zealand armies.

My first insight into the Irish in the first world war was through the eyes of the fictional character, Willie Dunne. The 2005 novel by Sebastian Barry, A Long Long Way, tells the story of a Dublin Fusiliers solider. It was fiction that taught me the facts of Irish history.

Next week, president Michael D Higgins will make the first visit by an Irish head of state to the UK. The four-day trip follows the successful visit by Queen Elizabeth and the Duke of Edinburgh to the Republic of Ireland in 2011, the first by the head of the British monarchy since 1911.

This normalisation of Anglo-Irish relations, 93 years after the Irish war of independence ended, is brimming with public symbolism and private emotion. On Wednesday morning, the president and his wife, Sabina, will be escorted by the Duke of York to the grand staircase in Windsor castle to view the colours of Irish regiments from the first world war – the Royal Dublin Fusiliers, Royal Irish Regiment, Royal Munster Fusiliers, Connaught Rangers, Prince of Wales's Leinster Regiment and the South Irish Horse, which were all disbanded following the establishment of the Irish Free State in 1922. The solitary act of standing before the colours will help Ireland to purposely remember what was deliberately forgotten.

Sylvester survived the war, but not the consequences of it. His wife died in September 1935 from meningitis. She had helped keep his shellshock at bay and he was entirely dependent on her support. He had lived with the noise of shelling in his head and the lingering taste of poison gas for 20 raw years. The memory of his daughter is one of her beloved father pacing the floor, over and over and over again.

"Suicide by gas poisoning, there being no evidence to show state of mind," read the death certificate, five months after his wife's death. On the back of a photograph of him taken after the war, are the words, "Dad died. We loved him."

My grandmother did not tell her children about the circumstances of her father's death and his service in the first world war until she was in her 70s. She did not want anyone to think badly of the father she loved. His final resting place lies outside Eccles near Manchester. "We let him be forgotten," she once whispered at his grave.

Thomas Kettle wrote a poem, To My Daughter Betty, the Gift of God. In that brutal intimacy at the front, did he show it to Sylvester, his fellow soldier in the Dublin Fusiliers? I dedicate this to my grandmother, who lost her father because of the war.

So here, while the mad guns curse overhead,

And tired men sigh with mud for couch and floor,

Know that we fools, now with the foolish dead,

Died not for flag, nor King, nor Emperor,

But for a dream, born in a herdsman's shed,

And for the secret Scripture of the poor.

• Dr Elaine Byrne is the author of Political Corruption in Ireland 1922-2010: A Crooked Harp?