Minister for Foreign Affairs Simon Coveney says Russia's announcement of a partial mobilisation of army reserves is "the most significant development" in the war since the Russian invasion started in February.
Mr Coveney will take part in a special meeting of the UN Security Council in New York tomorrow to discuss the Ukranian situation and expects a "very robust and blunt" debate.
Foreign minsters from Russia, Ukraine, the US, France and China will also take part.
He called the Russian plan to hold referenda in the four occupied Ukrainian provinces as a "sham", and said the world needs to respond "very firmly" to Russian threats.
Reacting to this morning's announcement by Russian President Vladimir Putin, Minister Coveney said: "What we've seen from President Putin is an escalation in terms of Russian intentions in the context of their war in Ukraine.
"They've announced a partial mobilisation, which effectively means that they intend to add about 300,000 people to their armed forces in the context of, of the ongoing war in Ukraine. That's not good news.
"It represents a significant escalation and if you add that to the Russian intention to hold what can only be described as sham referendums in occupied territories in Ukraine, to try to create some kind of false justification for escalating their aggression there, I think this signals a very unwelcome and worrying development in the context of the war, probably the most significant development actually since Russia's invasion in February."
This is a significant escalation in Russia's war on #Ukraine. A 'partial mobilisation’ in Russia will prolong this tragic, illegal conflict & increase deaths.— Simon Coveney (@simoncoveney) September 21, 2022
Sham referendums planned in occupied parts of Ukraine will never be recognised by Int community. https://t.co/dWn5g2ZLt2
Mr Coveney criticised the timing of the Russian move, which coincides with the UN General Assembly in New York, which is mainly focused on planning for a global food shortage next year.
In preview of his own address to the security council, Minster Coveney said: "Our contribution will be based as it always is on international law as our benchmark, the UN Charter as our benchmark, international crimes against humanity and war crimes as being the benchmarks that we will speak to.
"We, like many others in the international community, see this as Russian aggression in Ukraine that needs to be called out.
"If Russia decides to stop this war, well, then the killing would stop. And that is what needs to happen.
"So we'd approach this tomorrow by doing what we did, and have done in each of the Ukraine debates.
"We will talk about what is expected of countries in the world, particularly members of the UN Security Council in terms of how they behave in a way that is consistent with the UN Charter and international law.
"And of course, we will respond to us to what President Putin has said in the last 24 hours. Escalation is not what's needed here. The opposite is what's needed.
"We've already seen tens of thousands of people killed in this war, if not over 100,000 at this stage.
"And it should stop. And the UN Security Council is the place to call that out."
Mr Coveney said tomorrow will be the most important debate in Ireland's term on the security council.
He held a lengthy meeting with Ukrainian foreign minister Dymitro Kuleba in the Black Sea port of Odesa last week in preparation for the meeting, and said it was vital to listen to what the Ukrainian people and government have to say, as they are the country that has been invaded.
"We are guided by Ukraine, its president and people," he said.
Meanwhile, speaking on RTÉ's Drivetime, Minister of State Thomas Byrne said Mr Putin's nuclear threats were "a sign he is failing in his plans...and is going to prolong this tragic illegal conflict and increase suffering and deaths".
He said Ireland had been to the fore trying to bring on nuclear nonproliferation, making important points on the dangers and damage of it and reducing the number of nuclear weapons.