Pet form of Margaret, now also used as an independent given name. In the Middle Ages the short form Mag(g) was common, as a result of the early loss in pronunciation of the English preconsonantal r; it is no longer used as a given name, but it did give rise to the surname Maggs.
Irish: Anglicized form of Gaelic Ó Briain ‘descendant of Brian’, a personal name probably based on the element bre- ‘hill’, with the transferred sense ‘eminence’, i.e. ‘exalted one’. See also Bryan. In Ireland there has also been some confusion with O’Byrne (see Byrne).