Taoiseach Enda Kenny and Britain's Prince William have laid wreaths at the foot of the Round Tower memorial as part of the centenary commemorations for the Battle of Messines Ridge in Belgium.
They joined Princess Astrid of Belgium at the commemoration.
It marks the 100th anniversary of the start of the battle - which saw the 16th Irish Division and the 36th Ulster Division fight alongside each other for the first time during World War I.
Former Stormont first minister Arlene Foster was also among invited dignitaries.
A public ceremony took place at the Island of Ireland Peace Park in Messines at 2pm.
The park was built in 1998 - the year of the Good Friday Agreement - to mark the sacrifice of all Irishmen who fought and died in the war.
Mr Kenny and Prince William were to visit nearby Wytschaete Military Cemetery, where many of those who died were buried.
They paid their respects at the 16th Irish Division Memorial Cross in a private ceremony before meeting family members of servicemen buried in the cemetery.
The commemoration in Belgium was Mr Kenny's last overseas engagement as Taoiseach.
The village of Wytschaete was captured by the 36th and 16th during the Battle of Messines.
The capture of Messines Ridge was a key objective of the Allied Forces ahead of their mid-summer 1917 offensive on German lines between the nearby town of Ypres and the small village of Passchendaele - a battle that has since become synonymous with the horrors of trench warfare.
The Allies sustained around 10,000 casualties during the Battle of Messines, the Germans around 25,000.
Between them, the 36th and 16th lost around 2,500 men.
It was one of two occasions when Protestant and Catholic soldiers from Ireland fought side by side on the Western Front.
On the second occasion both sets of Irish soldiers fought together the result was not a triumph but a disaster.
That was the attack on Langemark two months later which pre-figured the ill-fated Passchendaele offensive, but it was marked by acts of incredible bravery, including the attempted rescue of Willie Redmond, the Irish Nationalist MP by a unionist private John Meeke.
