The Taoiseach has warned that Vladimir Putin will be held accountable for Russia's invasion of Ukraine with 'severe' sanctions.
Dozens of explosions have been reported across the Ukraine as Russian forces attack as part of an operation launched by President Putin overnight.
Within moments of confirming a military operation in the neighbouring nation, explosions rung out and could be heard across a number of eastern cities.
More than 40 people are said to have been killed so far.
Ukraine President Volodymyr Zelenskyy quickly announced martial law across all territories.
An emergency meeting of EU Leaders is set to take place on Thursday evening, where it is believed strong sanctions will be enforced on Russia.
Speaking outside Government buildings, Micheál Martin said: "Today's invasion and attack is an outrageous and moral breach of the most fundamental and basic principles of international law. Our thoughts must be with the innocent people on Ukraine. We stand with them.
"The UN Charter requires all countries to refrain from the threat or use of force against political independence and territory of any state. Today's use of brute force is a serious affront.
"It presents Europe with the most grave security situation it has faced in decades, with repercussions for global security. It cannot and will not go unanswered.
"President Putin and his henchmen will be held accountable. This evening I will join with our partners in the EU to adopt the most severe package of sanctions we have ever considered, on top of the measures already announced. They will be aimed to hit Putin and his henchmen where it hurts most.
"Actions have consequences and the consequences will be severe. They will weaken Russia's economic base. This aggression by Russia will cause untold hardship and tragedy to the people of Ukraine, who have shown remarkable resilience and resolve."
Mr Martin also noted that plans are in place for visa requirements for Ukrainian citizens to enter Ireland are set to be waived, with a meeting between Government ministers set for this afternoon.
