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 Ceallaigh ‘descendant of Ceallach’, an ancient Irish personal name, originally a byname meaning ‘bright-headed’, later understood as ‘frequenting churches’ (Irish ceall). There are several early Irish saints who bore this name. Kelly is now the most common of all Irish family names in Ireland.