Complex sentence: It is an anglicisation of the Gaelic surname "Ó Muiris", meaning'descendant of Muiris', which may be a variant of "Ó Muireasa" (anglicised as Morrissey), which was the name of a branch of the Uí Fiachrach.
Split into simpler sentences: It is an anglicisation of the Gaelic surname "Ó Muiris", meaning'descendant of Muiris'. The name may also be a variant of "Ó Muireasa" (anglicised as Morrissey), which was the name of a branch of the Uí Fiachrach.