Patrick Geoghegan is professor of modern Irish history in Trinity College and has also hosted a history show on Newstalk radio called “Talking History”.
It is understood that Mr Geoghegan is currently on sabbatical from Trinity College and will be given a leave of absence to join Mr Varadkar’s staff.
It is not yet known what salary he will be paid for the new position. Government sources said he will assist the Taoiseach with speechwriting, research and other issues.
Mr Varadkar was a guest at Mr Geoghegan’s wedding, held in the Trinity College chapel, last year.
According to the Trinity website, Mr Geoghegan is an “expert on the Anglo-Irish relationship in the late-18th and 19th centuries, as well as on the competing themes of constitutional nationalism and republicanism between 1782 and 1848”.
His biography also says he “has always been interested in the role of oratory in political debate and how oratory shaped political discourse”.
“His current work develops from these interests and examines how Edmund Burke both succeeded and failed in using oratory to change the nature of imperial debate in the 18th century,” it adds.
Mr Geoghegan is the author of a number of books, including a two-volume study of Daniel O’Connell: Liberator: The life and death of Daniel O’Connell and King Dan: The rise of Daniel O’Connell.
Mr Geoghegan did not return requests for comment.