The Foreign Affairs Minister has described what he saw during a recent visit to Ukraine as 'profoundly shocking'.
Updating the United Nations Security Council, Simon Coveney also made a direct appeal to Russia to agree an immediate humanitarian ceasefire.
It comes as Russia has launched an all-out assault on eastern Ukraine, according to the Ukrainian government - unleashing thousands of troops in what Kyiv described as the Battle of the Donbas.
The Kremlin has also declared that its main goal is to capture the Donbas region, after it failed to overrun Ukraine's capital.
Speaking in New York on Monday, Mr Coveney said Ireland will not remain silent.
"On Thursday of last week, I travelled to Kyiv.
"I did so because I wanted to see for myself the situation on the ground and to express Ireland’s solidarity with Ukraine and its people.
"What I saw was profoundly shocking.
"During my visit, I went to Bucha - until two months ago, it was a pleasant and vibrant town.
"A place where many of us around this table could have happily imagined living in.
"It now lies in ruins, with the stench of burning buildings and bodies in the air.
"I've been around long enough to know the difference between truth and staged propaganda when I see it.
"There was nothing fabricated about what I witnessed."
He also went into some graphic detail about what he saw.
"I stood at the edge of one of the mass graves, where the work of carefully exhuming bodies continued.
"503 civilians had been identified at that stage, and just four soldiers.
"503 individual human lives - men, women and children - who were not combatants, yet who appear to have been deliberately killed and in some cases after having been tortured in the most brutal manner."
'A war of choice'
He said the use of explosive weapons - including prohibited cluster munitions - being used in populated areas "speaks to an utter disregard by Russian forces for international humanitarian law and the protection of civilians."
Mr Coveney then spoke to the wider Security Council, suggesting: "Do we really have to keep repeating around this table that innocent civilians are never legitimate targets of war?
"That all parties to conflict must comply with international humanitarian law - including the prohibitions against indiscriminate and disproportionate attacks - and the obligation to distinguish between civilians and combatants?
"These obligations are not optional, not least for those privileged enough to be seated around this table, who should be leading by example."
And he said the current conflict is one of choice, that can end at any time.
"Ireland cannot, and will not, remain silent while this senseless and devastating war continues. Neither should any member of this council.
"As it was on the 25th of February, so it is today - this is a war of choice.
"And it can end immediately if [Russian] President Putin so decides.
"And yet, instead we are seeing a renewed offensive in eastern Ukraine.
"This is madness that history will judge harshly".
Mr Coveney added: "I want to call on Russia directly: agree to an immediate humanitarian ceasefire, commit to negotiations, and respect this Charter.
"We know that some progress was made in Istanbul between the parties on key issues.
"There is clearly a basis for a peace agreement."