McQuillan and MacQuillan are surnames of Irish origin. There are several unrelated origins of the surnames McQuillan and MacQuillan.
The Ulster variant of the surname was claimed to be an anglicisation of the Gaelic Mac Uighilín (son of Hugelin), the name allegedly adopted by the family of Hugelin de Mandeville. The de Mandevilles were a Cambro-Norman family and had conquered an area of north Antrim.
In reality the de Mandevilles sold their estates in northern Antrim to the McQuillans. Both families had previously held the office of "constable of the bonnaght" for the Earldom of Ulster – a system adopted from the Irish where mercenaries where hired to act as a body of standing troops. The McQuillans renamed the lands they acquired "the Route", derived from their "rout", a common term then for a private army.
These McQuillans played a large role in the history of County Antrim