The EU looks set to impose sanctions on Russia, after President Vladimir Putin declared he recognises two separatist regions in Ukraine as independent.
Mr Putin made the announcement during a live TV address tonight.
It comes after requests by the leaders of Luhansk and Donetsk - which broke away from Kyiv's control in 2014.
Recognition by the president of their independence could provide a pretext for Russian troops to cross the border into those areas.
And it could further narrow diplomatic options to avoid war, since it would be a rejection of a seven-year-old ceasefire brokered by France and Germany - which is still pushed as the framework for any future negotiations on the crisis.
European Commission President Ursula von der Leyen and Council President Charles Michel condemned the decree "in the strongest possible terms".
"This step is a blatant violation of international law as well as of the Minsk Agreements", the say in a joint statement.
"The union will react with sanctions against those involved in this illegal act.
"The union reiterates its unwavering support to Ukraine's independence, sovereignty and territorial integrity within its internationally recognised borders", they add.
In a statement, Foreign Minister Simon Coveney says this is something Ireland supports.
"Ireland's support for Ukraine's sovereignty and territorial integrity within its internationally recognised borders and its right to choose its own foreign and security policy path is unwavering.
"The decision by the Russian Federation to proceed with the recognition of the non-government controlled areas of the Donetsk and Luhansk oblasts of Ukraine as independent entities contravenes international law, is a blatant violation of Ukraine’s territorial integrity, and marks a clear and unilateral breach of the Minsk Agreements.
"Ireland supports a clear and strong EU response, including additional sanctions measures", he says.
He earlier described it as a "very negative development".