A number of commemoration events were held across France and the UK, including one at Belfast City Hall attended by the Secretary of State for Northern Ireland, Hilary Benn.
The first day of the Battle of the Somme, in northern France, was the bloodiest day in the history of the British Army and one of the most infamous days of the First World War.
On 1 July 1916, British forces suffered 57,470 casualties, including 19,240 fatalities.
More than 2,000 men from the 36th Ulster Division died in the first few days of the battle, which would last 141 days.
Thousands of men from what is now the Republic of Ireland also fought for the British Army in the ranks of the 16th Irish Division, which lost about 1,200 men in a single action during the Somme in September.
There were 420,000 British casualties in the battle alone – the price paid for advancing the front line by just four and a half miles (7.2 km).
