President Michael D Higgins has said a "great sense of darkness" has fallen across the world at the "unfolding tragedy" in Ukraine.
President Higgins said the hearts of Irish people go out to all of those who are suffering from this "completely unacceptable, immoral and unjustified violence".
Russia's invasion of Ukraine is now in its sixth day.
Satellite images show a huge build-up of Russian armoured vehicles and artillery 29km north of Kyiv, as the city braces for an all-out assault.
In a statement, President Higgins has urged Russia to withdraw its troops and end the rise of militarism.
"That we have returned to war, the abuse by the powerful of its neighbour, the flagrant violations of the principals of the United Nations, a great sense of darkness has fallen across the world at the unfolding tragedy in Ukraine," the President said.
"Our television screens carry images of all those mothers and babies crossing borders in order to flee the mayhem which is being inflicted upon them by an invading powerful neighbour operating with total disregard for the principles of international law, and of those brave Ukrainian people struggling to defend their homes and country.
"This violence must stop. Troops must be withdrawn by Russia. The rise of militarism must end. Full humanitarian access must be given to all civilians in need.
"Every glimmer of hope through diplomacy must be seized."
President Higgins called on "all those inflicting this violence" to reflect on the Universal Declaration of Human Rights, and its affirmation that "recognition of the inherent dignity, and of the equal and inalienable rights of all members of the human family, is the foundation of freedom, justice and peace in the world".
He said these are challenging times for diplomacy, multilateralism and international institutions.
It is essential, he said, that the peoples of the world come together and demand peace, describing it as "our best hopes for humanity".
"I call on everyone to use the tiny glimmers of hope which we have seen to bring this dreadful nightmare to an end and restore peace," President Higgins said.
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