The son of an Irish doctor and English Montessori teacher, Fanning was educated at St Michael's College, Dublin and C.B.C.

He received his undergraduate degree from UCD and his doctoral thesis on "Balfour and Unionism" from Cambridge University.

[3] He was Fulbright Professor at Georgetown University in Washington DC in 1976-1977, researching the triangular relationship between Britain, Ireland and the US.

Fanning's academic work focused primarily on diplomacy and high politics.

President Michael D Higgins described him as an "admired and respected historian whose extensive research and writings delivered a rich legacy to Irish scholarship".