He said the attacks were "calculated to sow fear at the heart of our societies"
President Michael D Higgins has condemned the recent attacks in Berlin, Zürich and Ankara.
12 people were killed when a truck ploughed into a Berlin Christmas market yesterday evening.
Interior minister Thomas de Maiziere said there was "no doubt" that it was a terror attack.
Officials have said they believe the attack was carried out by a migrant from Pakistan, but police have this afternoon said it is unclear if a man they have in custody was responsible. The arrested suspect has denied involvement.
Earlier yesterday evening, the Russian ambassador to Turkey, Andrey Karlov, was assassinated in the Turkish capital.
Mr Karlov was killed by off-duty policeman Mevlut Mert Altintas, who shouted "don't forget Aleppo" after shooting him.
In Zürich, three people were injured when a gunman attacked a mosque in the city. Police say they have recovered the body of a man believed to have carried out the attack.
In a statement, President Higgins said the attacks were "calculated to sow fear at the heart of our societies".
He said: "As President of Ireland I wish to convey my condolences to the families of the victims of these acts and the solidarity of the people of Ireland with all those affected by the violence. I wish all the survivors and their communities strength and confidence in these difficult times.
"Today, on this International Human Solidarity Day, let us redouble our efforts to celebrate and defend our unity in diversity so that we can, together, build a better tomorrow. We must strengthen our resolve not to let such attacks undermine the way of life in our global community, seeking to live in diversity and peace.”
In a statement last night, Taoiseach Enda Kenny said "these acts of violence have no place in society and constitute an attack on our fundamental values".