What is "gillick"

The surname (Mc)Gillick is a patronymic adopted by a branch of the Burkes of Connacht, and originates from the Irish Mag Uilic, meaning 'son of Ulick'. Mag is a form of Mac (son) used in old Irish names before vowels. William is Uilliam in Gaelic, and 'William the Younger' is Uilliam Og. As time passed, Uilliam Og was contracted to Uilleog, anglicized Ulick, which literally means 'young William', but has also come to mean 'little William'. The name Ulick came into use amongst the Burkes in the 14th century, and was originally peculiar to this family.