Taoiseach Simon Harris told delegates at a two-day peace summit in Switzerland that Ukraine "holds the same rights to territorial integrity" as any country.
More than 90 countries are attending the two-day talks at a Swiss Alpine resort at the behest of Ukrainian President Volodymyr Zelensky.
"It is important that we, as a global community, are clear about the principles of peace," said Mr Harris, adding that all nations represented at the summit have signed up today to the principles of the UN charter.
"In coming here today, we are saying that international law must prevail."
Seated next to Israel's foreign minister Yuli Yoel Edelstein, Mr Harris also emphasised the need for a ceasefire in Gaza.
This morning, Mr Harris took part in a plenary session on the issue of Ukrainian children illegally displaced by Russia.
He told delegates at the meeting that he was appalled by a report from the Ombudsman for Ukraine which detailed an incident of a Ukrainian child being interrogated, electrocuted and tortured after being abducted into Russia.
The Taoiseach said the world cannot allow this to go unchecked and reports of Russia's plans for more so-called child "vacations" this summer must be resisted.
"In a busy world where conflict competes for space, we must make sure there is that awareness right across the globe of these horrific war crimes against children," said Mr Harris.
A joint communiqué is expected to be issued this afternoon by the summit organisers detailing broad agreement from delegates on the areas of food security, nuclear safety and humanitarian issues such as the return of Ukrainian children to their country.
More than 90 countries are represented by national leaders and senior envoys at the summit, though Russia and China are not present.
Additional reporting: Liam Nolan
