The surname Heenan originated in Ireland but exactly when and how is a matter of uncertainty,  

The American Dictionary of Names states that HEENAN is an anglicized form of the Gaelic name Ó hÉanáin which means descendant of Éanán.

The evidence for this interpretation is not given but it bears a close resemblance to that of the Reverend Patrick Wolfe in Irish Names and Surnames published in 1923. He believes Heenan is a form of Ó hEidhneáin and is derived from a diminutive of eidhean meaning  ‘ivy’. Woulfe also suggests it may be a reduced version of Heneghan.

A different explanation for the surname origin is given by surnamedb.com which describes Heenan as an ancient Irish surname from the 13th century that is a nickname or metonymic. It supposedly owes its derivation from the early Gaelic word “ean” meaning a bird.

The origin, according to this source is the ancient name 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 according to surnamedb.com but it comments that it could indicate a hunter, hawker or a bird trainer in the service of a clan chief.

In The Surnames of Ireland, first published in 1983, Edward MacLysaght casts doubt on Wolfe’s idea of a connection between Heneghan and Heenan. Nor does he pick up on the idea Heenan derives from a  surname meaning descendant of the bird or one that is related to ivy. He does however connect it to the ancient clan-based social structure of Ireland.

He claims that it : “…, it is correctly the name of a small sept located in the Roscrea area on the border of north Tipperary and Offaly. It is also found in County Down. ”

The term ‘Sept’ is one that MacLysaght proposed should be used instead of ‘clan” when describing the historical social structure in Ireland,. He wanted to draw a distinction between the Irish system and the centralised Scottish clan system. In Irish Families, published in 1991 he argued that:

“The terms clan and sept have often been confused.  Ireland never had a well developed clan system like Scotland.   “Sept” is more appropriate as a collective term describing a group of persons or immediate ancestors who bore a common surname and inhabited the same territory.  Clan is more often used today as a generic term to indicate the family group. ”

Other genealogical and historical experts have disagreed with MacLysaght but the term sept has nevertheless become used to denote a sub group of a larger clan.

Variants of the Heenan surname

Among the variants of Heenan are: Henan, Hennan, Heanen,

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.