Mr Donnelly was speaking on July 1, having already attended the annual commemoration at the Knockagh Monument in East Antrim.
He added: “For the young men who were sent over the top to their deaths on the bloodiest day of the battle, we remember their sacrifice today.
“While the first day remains one of the most brutal in British Army history, over the course of the battle thousands of soldiers from both the 36th Ulster and 16th Irish divisions met their fates.
“It’s vital we remember that sacrifice, making commemoration events such as the Somme as inclusive as possible and affording all generations the opportunity to honour the young men who did not return from the battlefields.”