Opinions differ on the origin and meaning of the surname Heenan.
One source has it as an Anglicized form of Gaelic Ó hÉanáin meaning.’descendant of Éanán. Another source says it was thought to be a nickname or metonymic owing its derivation from the early Gaelic word “ean” meaning a bird.
According to the American Dictionary of Names HEENAN is an anglicized form of the Gaelic name Ó hÉanáin (descendant of Éanán). Woulfe, however, believes it to be a form of Ó hEidhneáin, which he derives from a diminutive of eidhean ‘ivy’. The same source indicates it may sometimes be a reduced version of Heneghan.
A different source (namely 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.
It derives from 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, 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.
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.