what does o stand for in irish names
Irish name A first name may be modified by an adjective to distinguish its bearer from other people with the same name. Mór ("big") and Óg ("young") are used to distinguish father and son, like English "junior" and "senior", but are placed between the given name and the surname: Seán Óg Ó Súilleabháin corresponds to "John O'Sullivan Jr." (although anglicised versions of the name often drop the "O'" from the name).
Monaghan (surname) The surname Monaghan /ˈmɒnəhæn/ is a family name originating from the province of Connacht in Ireland. Mostly a last name.