Ulster Unionists have rejected an Irish invitation to take part in commemorating the 1916 Easter Rising.
More than one million soldiers were killed at the Somme
Irish PM Bertie Ahern invited the party to participate in a joint celebration to mark both the 90th anniversary of the rising and the Somme offensive.
UUP assembly member Michael Copeland said the party regarded the rebellion, put down by the British, as "an act of terrorism during a time of war".
Mr Copeland said he preferred to commemorate relatives who died in WWI.
He said he respected the right of the people in the republic to "commemorate events in their history in any way they deem appropriate" and welcomed the steps being taken to honouring Irish soldiers who fought in WWI.
But he said the Dublin rising "heralded the end of the long and honourable tradition of constitutional Irish nationalism and brought to the fore the blood sacrifice ethos of armed republicanism".
Last month the Irish postal service, An Post, said it would launch a commemorative stamp to mark the 90th anniversary of the Somme - one of the bloodiest battles of World War I.
British forces put down the rebels in Dublin
Both the 36th (Ulster) and 16th (Irish) Divisions fought in the battle between 1 July and 13 November 1916.
By the end of the battle, the British suffered about 420,000 casualties, the French 195,000 and the Germans about 650,000.
Three months earlier, on 24 April, republicans seized control of parts of Dublin and declared Ireland a republic.
In a week of fighting between the rebels and British troops 450 people died, 250 of them civilians.