The Taoiseach has condemned "outrageous" actions of Russia in Ukraine and promised harsh new sanctions from the EU.
In a specially convened press conference at Government Buildings, Micheal Martin called the actions of Russia "an outrageous and moral breach of the most fundamental and basic principles of international law".
He said the EU would introduce "severe" sanctions against Russian leaders.
"Our thoughts must be with the innocent people of Ukraine at this, their most difficult hour.
"We stand with them."
He said Europe was dealing with the "most grave security situation it has faced in decades, with repercussions for global security and the world economy".
"It cannot and it will not go unanswered.
"President Putin and those who support him will be held accountable.
"This evening, I will join with our partners in the European Union in Brussels to adopt the most severe package of sanctions we have ever considered.
"They will be aimed to hit Putin and his henchmen where it hurts the most.
"EU sanctions will target strategic sectors of the Russian economy, blocking access to markets and to technologies."
"Assets in the EU will be frozen.
"They will have impacts in the EU, including in Ireland, and I will work with fellow leaders to support those hit hardest.
"This aggression by Russia will cause untold hardship and tragedy for the Government and people of Ukraine who have shown remarkable resilience and resolve.
"We will not be found wanting in offering them the humanitarian support they will need."
Mr Martin paid tribute to two Irish diplomats working in Kyiv and said that the Government was "maintaining close contact with Irish citizens in Ukraine".
He said that their safety is "paramount".
Mr Martin also said that the Ireland can especially understand the plight of Ukrainians.
"As a small country in particular, today's use of brute force in pursuit of a warped perception of national interest is a serious affront," he said.
Taking questions from reporters, Mr Martin said that he expected to find "unity" in Brussels over the necessary response.
He said that sanctions should not be "incremental" and needed to be "immediate".
Mr Martin said that the Minister for Foreign Affairs and the Minister for Justice were working together to start the process of waiving the visas requirements for all Ukrainian citizens entering Ireland.
"There will be a significant migration issue arising from these attacks. We will have to play our part in helping those who will have to flee Ukraine."
He said that any military response from the western countries was not something to be lightly considered.
"The economic sanctions and dealing with this in a different way from the military route is the most effective one to protect the lives of people across Europe."
Foreign Affairs Minister Simon Coveney earlier told RTE radio: "We are not going to go to war with Russia, militarily.
"I think the response in terms of sanctions will be very direct and very hard-hitting, and that means that the EU is also going to suffer as a result of those sanctions because undoubtedly there'll be counter-sanctions from Russia."
He indicated that it appears the time for diplomacy is now over.
"This is an illegal act of aggression by Russia," Mr Coveney said.
"They are lying to themselves and lying to the world about their justification for it."
Mr Coveney said it is not only an attack on Ukraine, but on the whole of Europe.
"It is also in many ways an attack on the kind of Europe we have built together, collectively, since World War Two."
The Department of Foreign Affairs has urged all remaining Irish citizens in Ukraine to seek shelter in a secure place.
It advised citizens not to move around the country in the coming hours and to follow the advice of local authorities.
Mr Coveney said two Irish families are currently in Ukraine for surrogacy services and that Irish officials are working with them.
The minister also confirmed that the Irish diplomatic team in Kyiv have moved to a safe place.
"Let's not forget what President Putin has done and said over the last number of hours is not only to start a war with Ukraine through a broad invasion, not just of the regions in the east of Ukraine but the country as a whole.
"But he's also warned any country that intervenes and helps Ukraine, he has also threatened them very likely.
"We are seeing a language that we simply have not seen in many, many decades in Europe of aggression and of potential brutality," Mr Coveney said.
He predicted a united response from the European Union and said that Ireland cannot remain politically neutral on such an issue.
"I think you will see extraordinary unity today from the European Union.
"Yes, Ireland is a neutral country.
"We're militarily non-aligned.
"But we are certainly not neutral on an issue like this, when there is blatant aggression happening on the continent of Europe."
Appearing on RTE radio, the Ukrainian Ambassador to Ireland said the international community needs to be "very vocal and very strong now".
Larysa Gerasko said: "I'm so emotional. I'm really shocked.
"My parents live with 150 kilometres from the northern border of Ukraine and they're shocked.
"We want to believe in the peaceful settlement and that Putin will stop on this point in Ukraine, and return to the roundtable of the negotiations, and all the international community has to be very vocal and very strong now."
