President Mary McAleese has laid a wreath at the war memorial in St Patrick's Cathedral in Dublin, during a Remembrance Sunday service in memory of those who died during active service with the British armed forces.
A wreath was also laid by the President of the Royal British Legion in Ireland.
The service was attended by the British Ambassador to Ireland, His Excellency Julian King, as well as hundreds of ex-servicemen and women from the British armed forces and Irish Defence Forces, relatives of deceased veterans and members of the public.
In Belfast, over 60 wreaths were laid at the cenotaph at City Hall and a two minute silence was observed.
The commemoration was led by the Lord Mayor Naomi Long.
It was also attended by Northern Ireland Security Minister Paul Goggins and the new PSNI Chief Constable Matt Baggott.
Wreaths were also laid on behalf of the governments of the United States, Canada, France and Poland.
Almost 50,000 people throughout Ireland were killed in WWI, while more than 5,000 Irish volunteers died in WWII.
