_Williamson._ _Mac Uilleam_, from _mac_, son; _Uilleam_, genitive of
_Uilliam_, William. Son of William. Fitzwilliam, Williams and MacWilliam
are forms of the word, the last being the Irish name assumed by the
Bourkes of Connaught on the death of their chief, William De Bourg, third
earl of Ulster.