Acts of remembrance have taken place in France marking the start of the Battle of the Somme 90 years ago.
More than one million soldiers were killed or wounded at the Somme
The 36th (Ulster) and 16th (Irish) Divisions were in the thick of the battle between 1 July and 13 November.
Thiepval Wood, where the Ulstermen went over the top, was officially dedicated to the memory of the soldiers.
Presbyterian Moderator David Clarke led the ceremony and said that it had already been hallowed by the courage of the men who "fought and died here".
"In this place, now a scene of beauty but once a place of mud and blood we meet to remember and give thanks," he said.
"We cannot truly consecrate this ground for it was hallowed long ago by brave men who fought and died here giving it a glory far beyond our poor power to add or detract."
On the first day alone of the battle more than 2,000 Ulstermen died. Before it ended more than one million British, French and German soldiers were killed or wounded.
NI Secretary Peter Hain and a number of local politicians took part in the ceremonies.
A cap and a poppy at the Somme Heritage Centre in County Down
"It is important to remember those who made the ultimate sacrifice. One the first day of the battle, in a country far from home, over 2,000 men from the 36th Ulster Division lost their lives and many more were wounded," he said.
"They were joined by thousands from across the island of Ireland and as the battle raged in the following weeks and months the losses endured by the Ulster and Irish divisions were particularly terrible.
"The bravery and sacrifice of all these men must never be forgotten and through these acts of remembrance taking place today, each and everyone of us will ensure that their memory is eternal."
The battle claimed the lives of 420,000 British soldiers.
French casualties were estimated at 195,000 and the German loss of life was about 650,000.
Soldiers of the Royal Irish Regiment and Royal Irish Rangers took part in the ceremonies.
Rangers formed guards of honour at the Ulster Memorial Tower and the 16th (Irish) Division memorial at the village of Guillemont.
More than 110,000 Irishmen from across the island gave their lives during World War I.
The Irish government was represented by minister Mary Hanafin.
Royal British Legion members in the Irish War Memorial Park
The Irish government held its own commemoration at the Irish National War Memorial at Islandbridge in Dublin.
This event was attended by southern politicians and by SDLP leader Mark Durkan and Ulster Unionist Alan McFarland.
Two representatives of the DUP, Lord Browne of Belmont and Jim Wells, also attended.
Ms Hanafin said the Dublin ceremony was a sign of changed times in Ireland which now looked "at all aspects of our history with maturity".
At the ceremony an Irish Defence Forces officer read the words of Marshal Foch, the overall commander of the allied forces at the Western Front, who paid tribute to the Irish.
"They have left an inspiration to duty that will live long after their names are forgotten," he said.
Marshal Foch said the words in 1928, at the 10th anniversary of the of the end of the war.
Then he said that France would "never forget her debt to the heroic Irish dead".
Sisters Yvonne Murray, 63, from Ratoath, Co Meath, and Kathleen Keating, 70, from Kimmage, Dublin, celebrated the life of their grandfather Patrick Murphy, who was wounded during the battle.
Peter Hain and Ian Adamson of the Somme Association at Thiepval
Mrs Murray, whose husband's grandfather was also killed in the battle, said: "It was a lovely service. It is never too late to remember our dead. Hopefully this will be held every year now."
Many of the men of the Ulster Division were members of the Orange Order and on Friday Orangemen laid wreaths at the Thiepval monument.
David Hay attended the ceremony and said that his father was among those who fought that day.
"My father was here and fortunately he got home," he said.
"But these men didn't get home. It's just emotional to be here and look at these graves and this is just one wee bit and how it's looked after - it's absolutely beautiful."