The Taoiseach, Mr Ahern, paid tribute to Ronald Reagan, saying that his interest in Irish affairs as US president was "invaluable" in the period leading up to the Anglo-Irish Agreement in 1985.
Mr Ahern said he learned of Mr Reagan's death on Saturday night with great sadness. His thoughts and prayers were with Mr Reagan's family and friends.
"I want to convey my sincere sympathies to his wife of more than 50 years, Nancy, and to his family. I also extend my sympathies to the American people," he said.
"President Reagan was proud of his Irish heritage and during his time in the White House, he celebrated St Patrick's Day with great ceremony. In 1984, President Reagan visited Ireland, an occasion still remembered with great fondness by many here."
Mr Ahern acknowledged Mr Reagan's role in the demise of communism and said this ultimately led to the EU's expansion into the former Eastern bloc.
"President Reagan was a determined opponent of communism and he played an important role in bringing an end to communism and to the artificial division of Europe imposed after the second World War. This process culminated in the accession of 10 new member-states to the European Union at the beginning of May this year."
