Private Thomas Chambers, known as Tommy, was among almost 20,000 British soldiers killed on July 1, 1916, little more than three months after he had arrived in France from Ireland on what he described as his "adventures".
His short diary, around 16 pages and written mostly in pencil, gives an insight into the preparations for a battle the British were hopeful they could win, but which resulted in a huge loss of life.
The young soldier's great-niece Helen McComb, from Glenanne, said she was moved to tears reading the last words of her relative who set off for battle never to return.
Ms McComb, who came across the diary recently when she heard there was to be a display for the centenary at the local Orange lodge, said: "My mother read it to us and told us about Tommy when we were children, and the diary was put away for safekeeping.
"But I hadn't realised the importance until recently."
The 56-year-old added: "I got goosebumps reading it. I just felt really emotional. I cried reading the last page. But I'm very proud that we have it and am happy for others to be able to see it."
While Pvt Chambers' gravestone records his age as 19, the soldier was just 16 when he signed up to the Army and 17 when he died fighting with the 36th Ulster Division, painstaking research by local woman Hilary Singleton uncovered.
The solicitor checked census records and looked into the history of what was happening in France on the dates of each of the diary entries.
Ms Singleton, who has written a play telling the story of Chambers and the other young men from the village of Glenanne who fought and died at the Somme, said: "It actually felt incredible to hold a diary that a soldier had in his breast pocket 100 years ago. It's in excellent condition."
Detailing his training and preparations for battle, Pvt Chambers' diary describes noisy shellfire, and marks out one specific bombardment by the British of the German front line as "like Hell let loose".
His final diary entry is brief but particularly haunting, as it is the only one made by the young soldier at the beginning of a day.
He writes simply: "Left for trenches at 2am 1st July."
Ms Singleton, who transcribed the passages as part of her research, said: "There's something completely unique about that, because every other entry he wrote at the end of the day. This entry was looking forward to what was about to happen.
"He's leaving for the trenches. Why did he write it like that?
"I think he wrote it that way because he thought it was likely that he was going to be killed. He had that level of realisation."
In the weeks before Chambers and his comrades would go over the top of their trenches to face a hail of German fire the men played sport - something Sergeant Joseph Lowry's grandson Colin said was particularly poignant to read.
Sergeant Lowry, also from Glenanne, was wounded in the shoulder on the first day of battle, but was one of the lucky ones who survived.
On returning home to a lack of employment the soldier, who was awarded the Military Cross, became disillusioned and threw a number of his war decorations into a nearby lake, his grandson said.
After reading Pvt Chambers' diary Mr Lowry (57), whose grandfather is mentioned by Tommy as having been with him at a football match in early June, said: "He (Tommy) really was only just a boy - they all were.
"It really brings it all home. They were playing a football match, all comrades, and in a matter of days they had died. It's really sad. And for the community, it would have been a big loss to their village at that time."
The diary will go on display at the Royal Irish Fusiliers Museum in Armagh as part of its Somme exhibition.
Riveting account of life in front line
Private Thomas Chambers served with the Royal Irish Fusiliers as part of the 36th Ulster Division. The teenager kept a diary called ‘Adventures from my leaving’.
Here are some extracts.
On April 3, he wrote: “Revolly (sic) blew 5.30. Breakfast 6.30. Parade 7.45, then marched to the training camp about twelve miles away carrying our dinner, sometimes two hard cakes, a piece of cheese and tea. We returned to at 4.30 every evening and had dinner which was very little.”
On April 22 he wrote: “Guard. At 9 o’clock at night our artillery started to bombard the German front line and it was just like Hell let loose. Shells bursting round me and machine gun fire.”
On April 30 Pte Chambers told how he “went to a service conducted by Rev Hallyhan and took holy communion under shellfire”.
German bombardment into the early hours of May 7 killed Pte Chambers’ Company Sergeant. He wrote: “We all had to commence rapid fire. The shells were flying every way, that heavy you could not have heard yourself speak.”
In his last letter home, dated June 30, Pte Chambers told his family he was in “very fair form” and promised to “write soon again”.
His final diary entry reads: “Left for trenches at 2am, 1st July.”