Ireland stands in full solidarity with the UK following the "loathsome and reprehensible" poisoning of ex-Russian spy Sergei Skripal and his daughter in Salisbury, the Taoiseach said.
The EU has urged Russia to address the UK's questions and called for full disclosure of its development of Novichok, the nerve agent used.
Leo Varadkar said it was important to stick together.
"An attempted assassination or a chemical attack, no matter who does it, is something that we're going to condemn and condemn outright," he said.
Mr Varadkar added that the 28 EU member states were all small countries surrounded by larger ones.
"As a fellow member of the European Union, we're going to be in full solidarity with the United Kingdom," he said.
"I think what happened at Salisbury was loathsome and reprehensible; and we're going to stand right beside the UK on that issue."
Mr Skripal, 66, and Yulia Skripal, 33, were found slumped and unconscious on a bench near a shopping centre in the centre of Salisbury on March 4, having visited a pub and a restaurant.
Prime Minister Theresa May blamed the Russian state.
Russia is on the agenda as leaders gather in Brussels for a European Council meeting.
President Donald Tusk has proposed strengthening resilience to chemical, biological, radiological and nuclear-related risks, including through closer cooperation among the EU institutions, the states and Nato.
He said: "We should also continue to bolster our capabilities to address hybrid threats, including in the areas of cyber, strategic communication and counter-intelligence."
European leaders are expected to adopt conclusions following the Salisbury attack, Mr Tusk added.