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.
German: from a Germanic personal name composed of the elements ragin ‘counsel’ + wald ‘rule’, the second element having been reinterpreted as hold ‘dear’, ‘beloved’ in the 16th century. This is also Jewish, presumably an adoption of the German surname.