From the name of a river in Ireland. It is not clear why it has become so popular as a given name; compare Clodagh. In part it may also be a transferred use of the Irish surname, Gaelic Ó Seanáin ‘descendant of Seanán’ (a diminutive of Seán). Shannon is not found as a traditional given name in Ireland itself.
Irish: Anglicized form of Gaelic Ó Díomasaigh ‘descendant of Díomasach’, a byname meaning ‘proud’, ‘haughty’, from díomas ‘pride’. The name was occasionally Anglicized as Proudman (see Proud). Compare McGimsey.