Gilligan
An Irish surname most often understood as a form of an older Gaelic personal-name family, likely tied to a diminutive nickname for a fair-haired or bright-headed ancestor.
The #5,557 most common surname in the United States (2010 Census) — about 1 in every 49,344 Americans shares it.
Etymology
Gilligan is an Anglicized Irish surname. The historical forms in the Irish sources point to a Gaelic name written approximately as O'Giollagáin / Ó Giollagáin, built on giolla, meaning 'lad,' 'youth,' or 'servant,' plus a diminutive ending. In older surname practice this usually signaled 'descendant of little Giolla-' or a related personal-name form, rather than a direct occupational meaning.
History
The name is found in Irish records and appears alongside related spellings such as Gillgan and Gilgan, showing that the English form was not fixed for a long time. It is especially associated with Irish families in the west and northwest, though later migration spread the surname much more widely. The dictionaries you provided treat it as an established Irish surname rather than as a recent or artificial formation.
Origin
Regions
Variant spellings
Fun fact
The same Gaelic root giolla turns up in many Irish surnames, which is one reason so many look similar at first glance.
From the historical record
“Glaspy. 407. GiLLIGAN. [Galligan] Qilgan. 709 GILLILAND. Gellaud. Gilelin. Gillan. 112. Gilland. Gilleland. Giililan.”
— Varieties and Synonymes of Surnames and Christian Names in Ireland