Taoiseach Micheál Martin has said it has been "an honour" to visit Hiroshima.
He told Japanese journalists: "I'm here first and foremost to remember, and to again sympathise on behalf of the Irish people, for all those who lost their lives during the atomic bombing and all those who suffered."
Mr Martin said he also wanted to reaffirm "… Ireland's commitment to nuclear disarmament and to nuclear Non-Proliferation."
Referring to Hiroshima Mayor Kazumi Matsui and his recent visit to Ireland, Mr Martin said the mayor told him that he found a great affinity with Ireland and the Irish people.
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Mr Martin said he backed the mayor and his international movement to form an alliance of cities dedicated to peace and to nuclear disarmament.
He laid a wreath at the cenotaph of the city's Peace Memorial Park, and rang the peace bell, which calls for all "nuclear arms and wars to be gone".
Mr Martin’s first stop off was at Hiroshima’s "Atomic Dome" - the only structure to survive the nuclear strike on the city in 1945.
6 August marks the 80th anniversary of the United States dropping the 4,400kg bomb.
It detonated 600m above Hiroshima, 43 seconds after having been dropped by the warplane Elona Gay, and set-off a fireball that devastated the city.
It is estimated the bomb immediately killed up to 70,000 people, and tens of thousands subsequently died in the radioactive aftermath of the explosion.
