The European Council president has said he does not like Brexit and "sometimes he is furious about it".
Donald Tusk told students in Dublin that the UK's decision to leave the EU was "one of the saddest moments in 21st Century European history".
Speaking on the 20th anniversary of the Good Friday Agreement, he also said people were anxious about the future of the peace process after Brexit.
The ex-Polish prime minister also spoke of his affection for Ireland.
Mr Tusk was speaking at a meeting organised by the University College Dublin Law Society.
Last month Mr Tusk said EU leaders wanted to see positive momentum in the negotiations to finalise a solution avoiding a hard Irish border.
The leaders agreed to proceed to the next stage of Brexit talks surrounding a transitional deal between the UK and EU.