Opinions differ on the derivation and meaning of the surname Heenan but there is agreement by all sources that the name originates in Ireland.
According to the American Dictionary of Names Heenan is an Anglicized form of the Gaelic name Ó hÉanáin (descendant of Éanán).
In Irish Names and Surnames, published 1903, the Rev Patrick Woulfe, described it as a form of Ó hEidhneáin, which he derives from a diminutive of eidhean, meaning ‘ivy’. He also indicated that Heenan is sometimes a reduced version of Heneghan.
A different source (surnamedb.com) describes Heenan as an ancient Irish surname from the 13th century. It is thought to be a nickname or metonymic, and to supposedly owe its derivation from the early Gaelic word “ean” meaning a bird.
This interpretation links Heenan to the ancient name of O’hEeanchain, which loosely translates as “The descendant of the son of the Bird”. Who the “Bird” was and how he became so named, is not known, but since it was normal for Gaelic surnames to descend from the original chief of the clan, it may be that the Bird was a hunter or hawker or a bird trainer.
Edward MacLysaght in his book Surnames of Ireland, finds the origin to be Ó hÉanáin. He goes on to say:
Although sometimes an abbreviated form of Heneghan, it is correctly the name of a small sept (a form of clan) located in the Roscrea area on the border of north Tipperary and Offaly. It is also found in County Down.
Early examples of variants of the Heenan surname
The first known recorded use of the name is believed to be that of Tayg O’Henehan, who was brought before the courts of County Kerry in the year 1295. His fate is not known. (source: http://www.surnamedb.com/Surname/Heenan#ixzz1DwvKsuJe
Thomas O’Henegan in Pettys “Survey of Ireland” for County Mayo in 1659
Father Patrick Henecan, the parish priest of Ballysodare, and Dean of Achonry in 1743.