His older brother Canon Maurice Hearne served as parish priest in Cahir, County Tipperary.

[6] The exact role of Hearne is not evident from the remaining documents; it is not clear who actually wrote the first draft,[7] and according to the Irish historian J. J. Lee, "Much remains to be uncovered about the planning and drafting of the constitution, including not least the roles of John Hearne, the legal adviser to External Affairs, and of Maurice Moynihan.

[9] It was Hearne who began the tradition of presenting a bowl of shamrock to the sitting U.S. President at the White House on St Patrick's Day.

[11] In 1960, after retiring from the diplomatic service, he became a legislative consultant to Nigeria and Ghana, which had recently acquired independence, albeit within the Commonwealth.

On the occasion of the 70th anniversary of the Irish constitution, Hearne's birthplace (8 William Street in Waterford) was marked with a tribute.