President Michael D Higgins has laid a wreath for Ireland's war dead during an interfaith service of commemoration in Dublin.
The Last Post was sounded and a single cannon shot fired in remembrance of men and women killed during armed conflict - including for Britain during the First World War and in service with the UN.
Irish premier Enda Kenny and ministers attended the ceremony at a former home for old soldiers.
The leader said: "It is fitting that we remember here today all those Irish men and women who died in past wars or in service with the UN.
"This year in particular we remember all those who died in the Great War."
Representatives of the Islamic, Christian and Jewish faiths gave readings about the importance of peace, on this centenary year of the First World War, at the Royal Hospital Kilmainham.
Founded in 1680, the Royal Hospital remained an old soldiers' home until 1927.
British Field-Marshal Sir John French, commander of the British Expeditionary Force, published a memorial book listing some 49,000 Irish soldiers who died during the First World War.
In recent years, as Anglo-Irish relations have markedly improved, there has been a growing acknowledgement in the Republic of Ireland of the contribution its citizens made fighting in the British army during the First World War - just before independence.
An official apology from the Irish government has also been issued to those thousands of soldiers who deserted its neutral forces during the Second World War to fight for Britain against Nazi Germany, and were effectively ostracised by the state on their return.
The commemorative ceremony featured martial pageantry and music. A band played a funeral march, the Celtic Lament, as well as Sibelius' Be Still My Soul.
The president inspected the army's guard of honour, standing in green ceremonial dress straight-backed in rows upon the cobbled stones of the Royal Hospital yard.
Mr Higgins laid a wreath, the Last Post sounded, a cannon shot was fired, a military fly past was held and the national anthem played.
Following the ceremony foreign affairs minister Charlie Flanagan said: "We have a human duty to remember that experience, and that loss.
"We have a national opportunity to understand those events better and to learn from them.
"And we have a collective obligation in commemorating them, to do so in a way that leaves each of us with a greater knowledge of ourselves, and a greater appreciation of the perspectives of others."
Ireland has a long history of service with difficult UN peacekeeping missions, including in Lebanon where 47 members lost their lives.
Minister for Trade and Development Joe Costello announced 1 million euros in funding to tackle violence against women and girls during contemporary conflicts or natural disasters.
He said: "Rape is often used in emergency and conflict situations to terrorise the population, break up families and destroy communities. Almost without exception, there is an increase in gender-based violence during and after natural and man-made disasters and crises."
The UN estimates that at least 200,000 women and girls n the Democratic Republic of the Congo have been the victim of rape since 1998.
Mr Costello added: "Violence prevents girls and women from reaching their potential and impoverishes individual women, their families and whole societies.
"The prevention of and response to violence against women and girls is in itself a life-saving action which needs to be an essential part of every humanitarian operation."